Projects that use pyx.build (ex. wendelin.core) need recent
setuptools_dso fixes:

https://github.com/mdavidsaver/setuptools_dso/issues/5
https://github.com/mdavidsaver/setuptools_dso/commit/67d717a6
https://github.com/mdavidsaver/setuptools_dso/commit/e40f5883
https://github.com/mdavidsaver/setuptools_dso/commit/40b492ab

Increase setuptools_dso version in pygolang's build requirement for
uniformity as well.

- 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)

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.