There's still more work to be done here of course. This
is just getting some initial files in place that will
eventually replace the scratch code and allow us to build
extensions.

For now, let's try and avoid the compile step because
it's one less thing we'll need to do in setup.py.  This
also makes it easier on a consumers and developers of Twisted
because there's less to setup on your local system just to run
the tests.

This change also removes the scratch file with some older code
in it.

This converts the internals of _pollingfile to
use the cffi implementation on win32.  There's some
new tests for win32 here too but _pollingfile itself
does not seem to have any direct tests.  It instead
is fed by other libraries which still need conversion.