This permits some more optimizations and speeds up the build another 20%
or so, in addition to being hopefully easier to follow.

On my machine, this cuts the build time by a fourth to a third.

Not ready to merge, submitting for testing on CI (esp Windows).

Cython calls are threaded because they release the GIL.

The most expensive part was merging results, which holds the GIL, so use
multiprocessing for that. Pre-combine the identical results that we
expect to get for two sets of defines to reduce the number of merges.

Change internal data structures to be immutable to make debugging this
easier (and substantially faster when pickling through multiprocessing.)

Don't subclass dict for environ by default, but leave that option
available. When printing, make sure that we have a secure environ.
Unfortunately, this is relatively fragile but it's the only way to not
break WebOb/Pyramid.

Right now, this is only used for suppressing the printing of potentially
sensitive information, but in the future there could be other uses.

This is technically not compliant with PEP3333 which specifies that the
type(environ) must be dict, but it's not clear if that practically
matters anymore (it looks like it might be a holdover from supporting
Python 1.5.2 before one could subclass the builtin dict; see https://mail.python.org/pipermail/web-sig/2003-December/000394.html).

Fixes #779.

test__socket_close specifies AF_INET sockets so make sure and use an AF_INET address in the server. This affects only Windows.

- Additional testing on a mix of ip4 and ip6 connections.
- Simplify address parsing.
- changelog.

Fixes #495.

That'll make the socket work both with IPv4 and IPv6.

Comments inline.

[skip ci]

We now depend on CFFI/Python new enough that it should be built during
setup/installation.

Trying to build at runtime easily results in "gevent" directories
scattered all over the system.

This makes the two branches equivalent in terms of scheduling. Fixes
notably test_ftplib.py which relies on running a "threaded" server.

- Compliance: If :func:`gevent.select.select` is given a negative *timeout*
  argument, raise an exception like the standard library does.
- Compliance: If :func:`gevent.select.select` is given closed or invalid
  file descriptors in any of its lists, raise the appropriate
  ``EBADF`` exception like the standard library does. Previously,
  libev would tend to return the descriptor as ready. In the worst
  case, this adds an extra system call, but may also reduce latency if
  descriptors are ready at the time of entry.
- Compliance: :meth:`gevent.select.poll.unregister` raises an exception if *fd* is not
  registered, like the standard library.
- Compliance: :meth:`gevent.select.poll.poll` returns an event with
  ``POLLNVAL`` for registered fds that are invalid. Previously it
  would tend to report both read and write events.

This serves to be better documenting about what they're doing and why,
instead of having a dozen slightly different loops that each have to be
examined.