chan<T> is a pointer type and e.g. send does not change the pointer - it
only "modifies" channel buffer. Marking appropriate methods as const is
needed so that `const chan<T>` could be used to send/receive the same
way as `chan <T>` is used.

ch->_mu was initialized with Sema - not Mutex - constructor.
It was appearing to work because currently Mutex is a trivial wrapper
around Sema.

As suggested by https://github.com/jayvdb run manifest checker and oops,
it finds that we forgot to include pyproject.toml for testprog/golang_pyx_user:

    missing from sdist:
      golang/pyx/testprog/golang_pyx_user/pyproject.toml
    suggested MANIFEST.in rules:
      recursive-include golang *.toml

-> Fix MANIFEST.in in generic way to include golang/*/*.toml

The bug was not affecting pygolang usage - only golang.pyx.build tests were
failing for sdist installed pygolang.

- Move channels implementation to be done in C++ inside libgolang. The
  code and logic is based on previous Python-level channels
  implementation, but the new code is just C++ and does not depend on
  Python nor GIL at all, and so works without GIL if libgolang
  runtime works without GIL(*).

  (*) for example "thread" runtime works without GIL, while "gevent" runtime
      acquires GIL on every semaphore acquire.

  New channels implementation is located in δ(libgolang.cpp).

- Provide low-level C channels API to the implementation. The low-level
  C API was inspired by Libtask[1] and Plan9/Libthread[2].

  [1] Libtask: a Coroutine Library for C and Unix. https://swtch.com/libtask.
  [2] http://9p.io/magic/man2html/2/thread.

- Provide high-level C++ channels API that provides type-safety and
  automatic channel lifetime management.

  Overview of C and C++ APIs are in δ(libgolang.h).

- Expose C++ channels API at Pyx level as Cython/nogil API so that Cython
  programs could use channels with ease and without need to care about
  lifetime management and low-level details.

  Overview of Cython/nogil channels API is in δ(README.rst) and
  δ(_golang.pxd).

- Turn Python channels to be tiny wrapper around chan<PyObject>.

Implementation note:

- gevent case needs special care because greenlet, which gevent uses,
  swaps coroutine stack from C stack to heap on coroutine park, and
  replaces that space on C stack with stack of activated coroutine
  copied back from heap. This way if an object on g's stack is accessed
  while g is parked it would be memory of another g's stack.

  The channels implementation explicitly cares about this issue so that
  stack -> * channel send, or * -> stack channel receive work correctly.

  It should be noted that greenlet approach, which it inherits from
  stackless, is not only a bit tricky, but also comes with overhead
  (stack <-> heap copy), and prevents a coroutine to migrate from 1 OS
  thread to another OS thread as that would change addresses of on-stack
  things for that coroutine.

  As the latter property prevents to use multiple CPUs even if the
  program / runtime are prepared to work without GIL, it would be more
  logical to change gevent/greenlet to use separate stack for each
  coroutine. That would remove stack <-> heap copy and the need for
  special care in channels implementation for stack - stack sends.
  Such approach should be possible to implement with e.g. swapcontext or
  similar mechanism, and a proof of concept of such work wrapped into
  greenlet-compatible API exists[3]. It would be good if at some point
  there would be a chance to explore such approach in Pygolang context.

  [3] https://github.com/python-greenlet/greenlet/issues/113#issuecomment-264529838 and below

Just this patch brings in the following speedup at Python level:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               20.0µs ± 1%  15.6µs ± 1%  -21.84%  (p=0.000 n=10+10)
    chan             9.37µs ± 4%  2.89µs ± 6%  -69.12%  (p=0.000 n=10+10)
    select           20.2µs ± 4%   3.4µs ± 5%  -83.20%  (p=0.000 n=8+10)
    def              58.0ns ± 0%  60.0ns ± 0%   +3.45%  (p=0.000 n=8+10)
    func_def         43.8µs ± 1%  43.9µs ± 1%     ~     (p=0.796 n=10+10)
    call             62.4ns ± 1%  63.5ns ± 1%   +1.76%  (p=0.001 n=10+10)
    func_call        1.06µs ± 1%  1.05µs ± 1%   -0.63%  (p=0.002 n=10+10)
    try_finally       136ns ± 0%   137ns ± 0%   +0.74%  (p=0.000 n=9+10)
    defer            2.28µs ± 1%  2.33µs ± 1%   +2.34%  (p=0.000 n=10+10)
    workgroup_empty  48.2µs ± 1%  34.1µs ± 2%  -29.18%  (p=0.000 n=9+10)
    workgroup_raise  58.9µs ± 1%  45.5µs ± 1%  -22.74%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               24.7µs ± 1%  15.9µs ± 1%  -35.72%  (p=0.000 n=9+9)
    chan             11.6µs ± 1%   7.3µs ± 1%  -36.74%  (p=0.000 n=10+10)
    select           22.5µs ± 1%  10.4µs ± 1%  -53.73%  (p=0.000 n=10+10)
    def              55.0ns ± 0%  55.0ns ± 0%     ~     (all equal)
    func_def         43.6µs ± 1%  43.6µs ± 1%     ~     (p=0.684 n=10+10)
    call             63.0ns ± 0%  64.0ns ± 0%   +1.59%  (p=0.000 n=10+10)
    func_call        1.06µs ± 1%  1.07µs ± 1%   +0.45%  (p=0.045 n=10+9)
    try_finally       135ns ± 0%   137ns ± 0%   +1.48%  (p=0.000 n=10+10)
    defer            2.31µs ± 1%  2.33µs ± 1%   +0.89%  (p=0.000 n=10+10)
    workgroup_empty  70.2µs ± 0%  55.8µs ± 0%  -20.63%  (p=0.000 n=10+10)
    workgroup_raise  90.3µs ± 0%  70.9µs ± 1%  -21.51%  (p=0.000 n=9+10)

The whole Cython/nogil work - starting from 8fa3c15b (Start using Cython
and providing Cython/nogil API) to this patch - brings in the following
speedup at Python level:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               92.9µs ± 1%  15.6µs ± 1%  -83.16%  (p=0.000 n=10+10)
    chan             13.9µs ± 1%   2.9µs ± 6%  -79.14%  (p=0.000 n=10+10)
    select           29.7µs ± 6%   3.4µs ± 5%  -88.55%  (p=0.000 n=10+10)
    def              57.0ns ± 0%  60.0ns ± 0%   +5.26%  (p=0.000 n=10+10)
    func_def         44.0µs ± 1%  43.9µs ± 1%     ~     (p=0.055 n=10+10)
    call             63.5ns ± 1%  63.5ns ± 1%     ~     (p=1.000 n=10+10)
    func_call        1.06µs ± 0%  1.05µs ± 1%   -1.31%  (p=0.000 n=10+10)
    try_finally       139ns ± 0%   137ns ± 0%   -1.44%  (p=0.000 n=10+10)
    defer            2.36µs ± 1%  2.33µs ± 1%   -1.26%  (p=0.000 n=10+10)
    workgroup_empty  98.4µs ± 1%  34.1µs ± 2%  -65.32%  (p=0.000 n=10+10)
    workgroup_raise   135µs ± 1%    46µs ± 1%  -66.35%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               68.8µs ± 1%  15.9µs ± 1%  -76.91%  (p=0.000 n=10+9)
    chan             14.8µs ± 1%   7.3µs ± 1%  -50.67%  (p=0.000 n=10+10)
    select           32.0µs ± 0%  10.4µs ± 1%  -67.57%  (p=0.000 n=10+10)
    def              58.0ns ± 0%  55.0ns ± 0%   -5.17%  (p=0.000 n=10+10)
    func_def         43.9µs ± 1%  43.6µs ± 1%   -0.53%  (p=0.035 n=10+10)
    call             63.5ns ± 1%  64.0ns ± 0%   +0.79%  (p=0.033 n=10+10)
    func_call        1.08µs ± 1%  1.07µs ± 1%   -1.74%  (p=0.000 n=10+9)
    try_finally       142ns ± 0%   137ns ± 0%   -3.52%  (p=0.000 n=10+10)
    defer            2.32µs ± 1%  2.33µs ± 1%   +0.71%  (p=0.005 n=10+10)
    workgroup_empty  90.3µs ± 0%  55.8µs ± 0%  -38.26%  (p=0.000 n=10+10)
    workgroup_raise   108µs ± 1%    71µs ± 1%  -34.64%  (p=0.000 n=10+10)

This patch is the final patch in series to reach the goal of providing
channels that could be used in Cython/nogil code.

Cython/nogil channels work is dedicated to the memory of Вера Павловна Супрун[4].

[4] https://navytux.spb.ru/memory/%D0%A2%D1%91%D1%82%D1%8F%20%D0%92%D0%B5%D1%80%D0%B0.pdf#page=3

Copy linux/list.h from wendelin.core which copied it from
util-linux.git which took it from linux.git at LGPL license state.

Here are corresponding links for wendelin.core and util-linux:

    nexedi/wendelin.core@a0f940ad
    https://git.kernel.org/cgit/utils/util-linux/util-linux.git/tree/include/list.h?id=v2.25-165-g9138d6f

Linked lists will be used in channels implementation in the next patch.

- Add semaphore alloc/free/acquire/release functionality to libgolang runtime;
- Implement semaphores for thread and gevent runtimes.

  * Thread runtime uses PyThread_acquire_lock/PyThread_release_lock +
    PyThread_acquire_lock/PyThread_release_lock, which, if used
    carefully, do not depend on GIL and on e.g. POSIX are tiny wrappers
    around sem_init(process-private) + sem_post/sem_wait(*).

  * Gevent runtime uses geven't Semaphore in Pyx mode.

- Add Sema and Mutex classes that use semaphores provided by a runtime
  in a RAII style.
- Add with_lock(mu) that mimics `with mu` in Python.

Sema and Mutex will be used in channels implementation in the followup
patch.

(*) during late testing a bug was found in CPython2 and PyPy semaphore
implementations on Darwin (technically speaking on POSIX with
_POSIX_SEMAPHORES undefined). Quoting the patch:

    FIXME On Darwin, even though this is considered as POSIX, Python uses
    mutex+condition variable to implement its lock, and, as of 20190828, Py2.7
    implementation, even though similar issue was fixed for Py3 in 2012, contains
    synchronization bug: the condition is signalled after mutex unlock while the
    correct protocol is to signal condition from under mutex:

      https://github.com/python/cpython/blob/v2.7.16-127-g0229b56d8c0/Python/thread_pthread.h#L486-L506
      https://github.com/python/cpython/commit/187aa545165d (py3 fix)

    PyPy has the same bug for both pypy2 and pypy3:

      https://bitbucket.org/pypy/pypy/src/578667b3fef9/rpython/translator/c/src/thread_pthread.c#lines-443:465
      https://bitbucket.org/pypy/pypy/src/5b42890d48c3/rpython/translator/c/src/thread_pthread.c#lines-443:465

    This way when Pygolang is used with buggy Python/darwin, the bug leads to
    frequently appearing deadlocks, while e.g. CPython3/darwin works ok.

    -> TODO maintain our own semaphore code.

So eventually we'll have push down and maintain our own semaphores,
at least for platforms we care, not to be beaten by CPython runtime bugs.

Move `ch = py{send,recv}.__self__` to right after case __class__ check.
For now both versions - old and new - can work, but when we'll move
channel implementations to C the new version will be required to access
pychan C-level attribute earlier than where ch is currently initialized.

For clarity to denote that things work at Python level:

    - casev	-> pycasev
    - case	-> pycase
    - recv	-> pyrecv
    - send	-> pysend

Channel object is still denoted as `ch` to reduce noise for when chan IO
code will be move into libgolang. `ch` will be renamed to `pych` after
that.

- put the logic to test-tweak what happens inside _blockforever into
  context manager pypanicWhenBlocked;
- place this manager in pyx code, where it can later be changed to tweak
  _blockforever at C level.

For clarity to distinguish where an object is Python-level channel, or
(later) a C-level channel.

Change `from golang import chan` to `from golang cimport pychan`; add
type annotations where pychan is used. Using pychan at C level will be
needed when test code will need to access C-level pychan attributes.

We will need to add C-level attributes to pychan and this requires it to
become cdef class. The class is exported because at least
_golang_test.pyx will also need to have access to those attributes.

If we just do `class pychan` -> `cdef class pychan` e.g. the following
starts to break:

    1.venv/local/lib/python2.7/site-packages/py/_path/local.py:701: in pyimport
        __import__(modname)
    golang/__init__.py:174: in <module>
        from ._golang import    \
    golang/_golang.pyx:455: in init golang._golang
        _pychan_send  = _pychan_send.__func__
    E   AttributeError: 'method_descriptor' object has no attribute '__func__'

and

    golang/_golang.pyx:513: in golang._golang.pyselect
        if im_class(recv) is not pychan:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    f = <built-in method recv of golang._golang.pychan object at 0x7f7055e57cc8>

        def im_class(f):
    >       return f.im_class
    E       AttributeError: 'builtin_function_or_method' object has no attribute 'im_class'

    golang/_pycompat.py:28: AttributeError

This is probably because for `cdef class` methods Cython does not
emulate full method bindings the same way as Python does.  Anyway we can
check which method is passed to pyselect by chanop.__name__ or by
inspecting PyCFunction directly. And not having method binding wrapper
should only remove a bit of overhead.

So we are ok with reworking send/recv chanop detection, and since
this way im_class provided by golang._pycompat becomes unused, it is
also removed.

The timings are probably within noise:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               21.7µs ± 1%  20.0µs ± 1%  -7.60%  (p=0.000 n=10+10)
    chan             9.91µs ± 4%  9.37µs ± 4%  -5.39%  (p=0.000 n=10+10)
    select           19.2µs ± 4%  20.2µs ± 4%  +5.62%  (p=0.001 n=9+8)
    def              58.0ns ± 0%  58.0ns ± 0%    ~     (all equal)
    func_def         44.4µs ± 0%  43.8µs ± 1%  -1.22%  (p=0.000 n=10+10)
    call             63.0ns ± 0%  62.4ns ± 1%  -0.95%  (p=0.011 n=10+10)
    func_call        1.05µs ± 1%  1.06µs ± 1%    ~     (p=0.059 n=10+10)
    try_finally       135ns ± 0%   136ns ± 0%  +0.74%  (p=0.000 n=10+9)
    defer            2.36µs ± 1%  2.28µs ± 1%  -3.59%  (p=0.000 n=10+10)
    workgroup_empty  49.0µs ± 1%  48.2µs ± 1%  -1.63%  (p=0.000 n=10+9)
    workgroup_raise  62.6µs ± 1%  58.9µs ± 1%  -5.96%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               21.7µs ± 1%  20.5µs ± 1%  -5.33%  (p=0.000 n=10+9)
    chan             9.91µs ± 4%  9.72µs ± 5%    ~     (p=0.190 n=10+10)
    select           19.2µs ± 4%  19.5µs ±14%    ~     (p=0.968 n=9+10)
    def              58.0ns ± 0%  58.0ns ± 0%    ~     (all equal)
    func_def         44.4µs ± 0%  45.4µs ± 1%  +2.23%  (p=0.000 n=10+10)
    call             63.0ns ± 0%  64.0ns ± 0%  +1.59%  (p=0.000 n=10+10)
    func_call        1.05µs ± 1%  1.06µs ± 0%  +0.65%  (p=0.002 n=10+10)
    try_finally       135ns ± 0%   137ns ± 0%  +1.48%  (p=0.000 n=10+10)
    defer            2.36µs ± 1%  2.38µs ± 1%  +0.72%  (p=0.006 n=10+10)
    workgroup_empty  49.0µs ± 1%  48.2µs ± 1%  -1.65%  (p=0.000 n=10+10)
    workgroup_raise  62.6µs ± 1%  60.3µs ± 1%  -3.69%  (p=0.000 n=10+10)

For consistency to denote that this functions work at Python level:

    - len_sendq    -> pylen_sendq
    - len_recvq    -> pylen_recvq
    - waitBlocked  -> pywaitBlocked

Plain test code movement with s/panic/pypanic/ as in golang.pyx panic is
already there and means semantically different thing. We need this code to
live in pyx world, because when channels implementation will be moved to
C, this utilities will need to be adjusted in a way that is not possible
to do from Python.

Denote what a method returns via `# -> ...` suffix.

Use `ch` instead of `self` for pychan methods. This aligns with how
(py)chan objects are currently denoted in (py)select and will reduce the
difference when chan code is moved into libgolang. We are sticking to Go
convention here.

After the code with channels implementation is moved into libgolang, the
rest bits in golang.pyx will be changed to refer to pychan objects as
pych for clarity.

To denote that this function/classes work at Python level:

    - chan    -> pychan
    - select  -> pyselect
    - default -> pydefault
    - nilchan -> pynilchan

Plain code movement with just s/panic/pypanic/ as in golang.pyx panic is
aleady there and means semantically different thing. Moved code, even
though it lives in golang.pyx, is still Python code and requires Python
runtime and GIL. We'll be splitting channels implementation into nogil
world in the following patches.

Just plain movement to Cython brings the following speedup:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               26.6µs ± 1%  21.7µs ± 1%  -18.54%  (p=0.000 n=10+10)
    chan             13.7µs ± 1%   9.9µs ± 4%  -27.80%  (p=0.000 n=10+10)
    select           29.3µs ± 2%  19.2µs ± 4%  -34.65%  (p=0.000 n=9+9)
    def              55.0ns ± 0%  58.0ns ± 0%   +5.45%  (p=0.000 n=10+10)
    func_def         44.0µs ± 1%  44.4µs ± 0%   +0.72%  (p=0.002 n=10+10)
    call             64.0ns ± 0%  63.0ns ± 0%   -1.56%  (p=0.002 n=8+10)
    func_call        1.09µs ± 1%  1.05µs ± 1%   -2.96%  (p=0.000 n=10+10)
    try_finally       139ns ± 2%   135ns ± 0%   -2.60%  (p=0.000 n=10+10)
    defer            2.36µs ± 1%  2.36µs ± 1%     ~     (p=0.617 n=10+10)
    workgroup_empty  58.1µs ± 1%  49.0µs ± 1%  -15.61%  (p=0.000 n=10+10)
    workgroup_raise  72.7µs ± 1%  62.6µs ± 1%  -13.88%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               28.6µs ± 0%  25.4µs ± 0%  -11.20%  (p=0.000 n=8+9)
    chan             15.8µs ± 1%  12.2µs ± 1%  -22.62%  (p=0.000 n=10+10)
    select           33.1µs ± 1%  23.3µs ± 2%  -29.60%  (p=0.000 n=10+10)
    def              55.0ns ± 0%  56.0ns ± 0%   +1.82%  (p=0.000 n=10+10)
    func_def         44.4µs ± 2%  43.0µs ± 1%   -3.06%  (p=0.000 n=10+9)
    call             64.0ns ± 2%  69.0ns ± 0%   +7.81%  (p=0.000 n=10+10)
    func_call        1.06µs ± 0%  1.06µs ± 1%     ~     (p=0.913 n=8+9)
    try_finally       136ns ± 0%   139ns ± 0%   +2.21%  (p=0.000 n=9+10)
    defer            2.29µs ± 1%  2.38µs ± 2%   +3.58%  (p=0.000 n=10+10)
    workgroup_empty  73.8µs ± 1%  70.5µs ± 1%   -4.48%  (p=0.000 n=10+10)
    workgroup_raise  94.1µs ± 0%  90.6µs ± 0%   -3.69%  (p=0.000 n=10+10)

- Add go functionality to libgolang runtime;
- Implement go for thread and gevent runtimes.

  * Thread runtime uses PyThread_start_new_thread which, if used
    carefully, does not depend on Python GIL and on e.g. POSIX reduces to
    tiny wrapper around pthread_create.

  * Gevent runtime uses gevent's Greenlet in Pyx mode. This turns
    gevent to be a build-time dependency.

- Provide low-level _taskgo in C client API;
- Provide type-safe C++-level go wrapper over _taskgo;
- Switch golang.go from py implementation into Pyx wrapper over
  Pyx/nogil API.

This is the first patch that adds Pyx/C++/C-level unit tests and hooks
them into golang_test.py .

This patch brings the following speedup to Python-level interface:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               93.0µs ± 1%  26.6µs ± 1%  -71.41%  (p=0.000 n=10+10)
    chan             13.6µs ± 2%  13.7µs ± 1%     ~     (p=0.280 n=10+10)
    select           29.9µs ± 4%  29.3µs ± 2%   -1.89%  (p=0.017 n=10+9)
    def              61.0ns ± 0%  55.0ns ± 0%   -9.84%  (p=0.000 n=10+10)
    func_def         43.8µs ± 1%  44.0µs ± 1%   +0.66%  (p=0.006 n=10+10)
    call             62.5ns ± 1%  64.0ns ± 0%   +2.40%  (p=0.000 n=10+8)
    func_call        1.06µs ± 1%  1.09µs ± 1%   +2.72%  (p=0.000 n=10+10)
    try_finally       137ns ± 0%   139ns ± 2%   +1.17%  (p=0.033 n=10+10)
    defer            2.34µs ± 1%  2.36µs ± 1%   +0.84%  (p=0.015 n=10+10)
    workgroup_empty  96.1µs ± 1%  58.1µs ± 1%  -39.55%  (p=0.000 n=10+10)
    workgroup_raise   135µs ± 1%    73µs ± 1%  -45.97%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               68.8µs ± 1%  28.6µs ± 0%  -58.47%  (p=0.000 n=10+8)
    chan             14.8µs ± 1%  15.8µs ± 1%   +6.19%  (p=0.000 n=10+10)
    select           32.0µs ± 0%  33.1µs ± 1%   +3.25%  (p=0.000 n=10+10)
    def              58.0ns ± 0%  55.0ns ± 0%   -5.17%  (p=0.000 n=10+10)
    func_def         43.9µs ± 1%  44.4µs ± 2%   +1.20%  (p=0.007 n=10+10)
    call             63.5ns ± 1%  64.0ns ± 2%     ~     (p=0.307 n=10+10)
    func_call        1.08µs ± 1%  1.06µs ± 0%   -2.55%  (p=0.000 n=10+8)
    try_finally       142ns ± 0%   136ns ± 0%   -4.23%  (p=0.000 n=10+9)
    defer            2.32µs ± 1%  2.29µs ± 1%   -0.96%  (p=0.000 n=10+10)
    workgroup_empty  90.3µs ± 0%  73.8µs ± 1%  -18.29%  (p=0.000 n=10+10)
    workgroup_raise   108µs ± 1%    94µs ± 0%  -13.29%  (p=0.000 n=10+10)

(small changes are probably within noise; "go" and "workgroup_*" should be
representative)

The next patch will add `go` to Pyx API and correspondingly C, C++ and Pyx
level tests for it that will go into _golang_test.pyx extension. For
C-level test we'll use C source file - to verify that libgolang.h could
be used by C projects with C compiler, while for C++ and Pyx-level tests
the sources will be in C++. Thus _golang_test.so will be build from both
C and C++ sources.

This creates a problem: distutils / setuptools use the _same_ compiler
to compile both C and C++ sources and only use C++ compiler at link
stage. Thus, as it is not possible to tune compiler that is used only
for C++ sources, and also as it is not possible to provide per-source
flags, when compiling C-level test, the compiler will be invoked with
`-std=c++11` option that we inject. Gcc tolerates that and only prints a
warning, but Clang considers that an error and gives:

	error: invalid argument '-std=c++11' not allowed with 'C'`

A proper fix would be to change the build system from distutils to
something more flexible that uses C++ compiler for C++ sources and C
compiler for C sources and allows to tune per-unit flags. However it is
not a small step at this stage.

-> Use workaround and tweak options that are used when compiling sources
depending on whether it is C or C++.

- Add sleep functionality to libgolang runtime;
- Implement sleep for thread and gevent runtimes. Thread runtime
  implements sleep independently of GIL, but only for POSIX for now;
- Switch golang.time py module into using golang.time pyx module.

As we are adding sleep, related functionality to query system about
"what is current time?" is also added.

Libgolang will be generic library and it will adapt itself to particular
execution environment by way of runtime plugin. This commit introduces
stubs for two such runtimes - "thread" and "gevent" - and initializes
libgolang with particular runtime depending on whether golang is
imported with gevent preactivated or not.

The runtimes themselves are empty at this step. We'll be adding runtime
functionality in the following patches.

Start transforming Pygolang from being pure Python project into project
that uses both Python and Cython and provides both Python and Cython API
to its users.

For Cython API there is a goal for it to be usable independently of
Python GIL - i.e. independently of Python runtime and limitations
imposed by Python's global interpreter lock. The motivation for this
work is wendelin.core v2, which, due to its design, would deadlock if it
tries to take the GIL in its pinner thread.

This commit brings bare minimum establishing libraries and build system.
It:

- Introduces C/C++ library libgolang. Libgolang will be serving most of
  nogil functionality and Pyx API will be a small wrapper over it. At
  this step Libgolang comes only with C-level panic;

- Introduces Pyx package golang that is aliased to Pyx package
  golang._golang . Users can do `from golang cimport ...` similarly to how
  they do py-level `from golang import ...`. At this step golang.pyx
  only wraps panic and provides way to transform C-level panic into Python
  exception;

- Introduces Py package golang.pyx.build . This package, similarly to
  golang.org/pkg/go/build, serves as the system to build Pyx packages -
  both external packages that use Pygolang in pyx mode, and to build
  Pygolang itself. The build, internally, is served by setuptools_dso
  and cython. An example how to use golang.pyx.build from external
  project comes in golang/pyx/testprog/golang_pyx_user/ and is used by
  tests to verify golang.pyx.build functionality;

- Introduces Pygolang build dependency to cython and setuptools_dso.

We can include *.py from top-level directories, not only under
X/testdata/, X/testprog/ ...

This test is veriyfing only how leaked goroutines behave, not go in
general.

test_go was doing the same what pyrun is doing + adjusting PYTHONPATH a
bit. Merge PYTHONPATH adjustment into pyrun and use pyrun uniformly.

pyrun runs command without piping output.
pyout runs command and returns its output.

We'll need to use this utility in other places; golang_test is the
central place for testing utilities.

gpython/gpython_test.py:23: 'subprocess' imported but unused
gpython/gpython_test.py:24: 'six.PY3' imported but unused

... and everything that whole pygolang depends on in pygolang[all].

The reason we do this: currently golang.perf.benchlib needs NumPy, but
it would be not ok to depend whole pygolang on numpy. Similarly pybench
needs pytest, but it would be not ok to require pytest unconditionally.

Thus allow users to specify what they need from pygolang via extras,
like pygolang[x.perf.benchlib] or pygolang[cmd.pybench], with
pygolang[all] serving as merge point for all subpackages dependencies.

TODO in the future we'll, hopefully, also provide accumulation points,
like pygolang[x.perf] which contains everything pygolang[x.perf.*]
depends on.

Note: core functionality and gpython are provided always unconditionally.

This test passes, but the functionality was not tested before. In
particular all tests were passing even with the following hand-edits:

    --- a/golang/__init__.py
    +++ b/golang/__init__.py
    @@ -627,7 +627,7 @@ def selected():
             sel = g.which
             if isinstance(sel, _SendWaiting):
                 if not sel.ok:
    -                panic("send on closed channel")
    +                panic("send on closed channel ZZZ")
                 return sel.sel_n, None

             if isinstance(sel, _RecvWaiting):

with added tests the bug is caught and reported:

    def test_select():
        N = 1000 # times to do repeated select/chan or select/select interactions

        # sync: close vs select(send)
        ch = chan()
        def _():
            waitBlocked(ch.send)
            ch.close()
        go(_)
>       with panics("send on closed channel"): select((ch.send, 0))

golang_test.py:353:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <golang.golang_test.panics instance at 0x7fc1da66e5f0>, exc_type = <class 'golang._PanicError'>, exc_val = _PanicError('send on closed channel ZZZ',)
exc_tb = <traceback object at 0x7fc1dabc33b0>

    def __exit__(self, exc_type, exc_val, exc_tb):
        ok = self.raises.__exit__(exc_type, exc_val, exc_tb)
        if not ok:
            return ok
        # _PanicError raised - let's check panic argument
>       assert self.exc_info.value.args == (self.arg,)
E       AssertionError: assert ('send on clo...channel ZZZ',) == ('send on closed channel',)
E         At index 0 diff: 'send on closed channel ZZZ' != 'send on closed channel'
E         Use -v to get the full diff

golang_test.py:1032: AssertionError

For buffered channel _tryrecv, on success, was unlocking ch._mu too
early - before accessing ch._dataq with ch._dataq.append().

Without the fix, newly added test breaks as e.g.

    golang/golang_test.py::test_chan_buf_recv_vs_tryrecv_race Exception in thread Thread-3:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
        self.run()
      File "/usr/lib/python2.7/threading.py", line 754, in run
        self.__target(*self.__args, **self.__kwargs)
      File "/home/kirr/src/tools/go/pygolang-master/golang/golang_test.py", line 317, in _
        assert (_, _rx) == (1, None), ('i%d' % i)
    AssertionError: i30
    assert (0, None) == (1, None)
      At index 0 diff: 0 != 1
      Full diff:
      - (0, None)
      ?  ^
      + (1, None)
      ?  ^

For buffered channel _trysend, on success, was unlocking ch._mu too
early - before accessing ch._dataq with ch._dataq.popleft().

Without the fix, newly added test breaks as e.g.

    golang/golang_test.py::test_chan_buf_send_vs_tryrecv_race Exception in thread Thread-3:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
        self.run()
      File "/usr/lib/python2.7/threading.py", line 754, in run
        self.__target(*self.__args, **self.__kwargs)
      File "/home/kirr/src/tools/go/pygolang-master/golang/golang_test.py", line 256, in _
        assert (_, _rx) == (1, None)
    AssertionError: assert (0, 209) == (1, None)
      At index 0 diff: 0 != 1
      Full diff:
      - (0, 209)
      + (1, None)

    Exception in thread Thread-2:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
        self.run()
      File "/usr/lib/python2.7/threading.py", line 754, in run
        self.__target(*self.__args, **self.__kwargs)
      File "/home/kirr/src/tools/go/pygolang-master/golang/golang_test.py", line 243, in _
        ch.send(i)
      File "/home/kirr/src/tools/go/pygolang-master/golang/__init__.py", line 340, in send
        ok = self._trysend(obj)
      File "/home/kirr/src/tools/go/pygolang-master/golang/__init__.py", line 417, in _trysend
        rx = self._dataq.popleft()
    IndexError: pop from an empty deque

Currently it behaves correctly, but this aspect will need to be
explicitly cared about when channel implementation moves to C. Add test
to make sure it won't regress.

Starting from b51b8d5d (select: Run tests more thoroughly) we are
running select subtests in repeated mode with N=1000. However not all
select subtests were run in repeated mode - for example e.g.
"non-blocking try send: not ok" was being run only once.

-> Rework all select subtests to be run in repeated mode to increase the
probability of catching bugs.

Channels implementation will soon be moved to C and will become
independent from Python runtime. This way while pygolang will still be
providing panic, hooking in and raising arbitrary Python-level exception
will become problematic. -> Rework "blocks forever" tests to rely on
just panic to prepare for that.

Chan implementation is going to be moved into C, and this way direct
access to chan internals won't be possible. C implementation will export
dedicated functions for chan tests which will be used.

Prepare for that and factor out retrieving len of chan recv/send queues
into separate functions, which will be adapted at the time of C port.