This eliminates our sdist-time dependency on Makefile or make.cmd on
Windows. Fewer moving pieces are better.

Fixes #1076

Update corecext.ppyx

This eliminates the need to call cythonpp.py on it. It also leads to a
more consistent API.

We could take the same approach to bring back sig_fd if necessary, but
that's still not available to cython.

Attempt to workaround Visual C 9.0 for CPython 2.7 on windows missing stdint.h. Also correct a compiler warning.

We should be able to use dummy functions to link the child watchers on windows, and forbid them from being created. This gets us down to only one CPP def, LIBEV_EMBED

cython ignores the preprocessor directives we previously had in
libev.pxd, so that was doing nothing, and it was treating vfd_socket_t
just as something it had to turn into an int.

The function it used to do this was the same function it used to turn
a generic PyObject into an int, once all the typedefs were substituted
in (which does use the c preprocessor from libev_vfd.h). So it should
be functionally equivalent.

The vfd_* functions are no-op macros on POSIX, so there shouldn't be a
performance difference. The minor exception is that __dealloc__ is
always defined, and although it's a static function suitable to
inline, there is an extra Python API call to PyErr_Fetch and
PyErr_Restore. But if that makes a difference, I didn't spot it.

Move to the safer __cinit__ for all watchers.

Now that the base watcher has access to its libev ev_watcher pointer, start moving the properties there, starting with ref.

We no longer need to gc before getting a libuv io object on PyPy: everything is deterministically closed (or should be).

Debugging for a weird EPERM error in test__fileobject.py on travis with libuv; not seen locally or on appveyor [skip appveyor].

And must be close() when done. The IO watchers already required this
for libuv; doing it for everything simplifies memory management and
object lifetimes and lets us get rid of a bunch of weakref objects
with callbacks.

This is a new requirement and will only affect those that use
CFFI (e.g., PyPy) and specifically it's needed for libuv; libev
doesn't really do much with it.

Some debugging help for forgetting to close objects piggybacks on
tracemalloc on Python 3.

test_ftplib.py fails in Python 3 because of some warnings about
unclosed objects.
test__backdoor.py fails in Python 2 for the same reason.

[skip appveyor]