Here is a diff against the previously posted _pickle.c module. The
module is pretty much done now. I fixed find_class() to be a proper
method and move the initialization code of Pickler/Unpickler into the
tp_init slot to allow proper subclassing.

The only things I didn't do is make save_reduce() public (since it would
be useless) and fix instantiate() to be exactly the same as in pickle.py
(since I have no idea how to cleanly inject a __class__ attribute into a
pre-build empty class from C).

With the beta postponed, I will probably add some optimizations to the
module and update the stdlib documentation.