Performance and correctness improvements for Greenlets.

- Rework getting the stack, since we're no longer arbitrarily extending it. This is faster, and I found and fixed a bug on CPython.
- Let Cython do more of the work making sure we really have a hub as our parent.

Update process.py example to work on windows.

Upgrade to PyPy3.6-7.1. Fixes #1373.

The "Unable to run on windows" warning is actually about uname/ls, not
about gevent :)

libev-cffi: Use nlink_t and let the compiler fill in the definition.

Fixes #1372.

This may require a slightly stricter adherance to POSIX? It is defined by IEEE Std 1003.1 2004 (http://pubs.opengroup.org/onlinepubs/009696699/basedefs/sys/types.h.html)

Update to PyPy 2 7.1

Also restore the ban on test_ssl.py: the allocation failures still exist.

Fixes #1373

Avoid unbounded memory usage in deep spawn trees. Fixes #1371.

Avoid a memory leak when wrapping an io.BufferedWriter around a socket.

Implement the fix in Cython so we don't lose any speed (we actually benchmark a bit faster).

Fixes #1318 and fixes #1319

 Patch the 'thread' module provided by 'future' if it's already imported

Patch the 'thread' module provided by 'future' if it's already imported, which is likely because pkg_resources imports it if installed.

Also some more cleanup from removing py3.4 support.

Took some tweaking of connection params --- did something change in OS X? And PyPy's GC has apparently changed the way it deals with sockets, we're better off being fully explicit about lifetime.