Simple sq_item for testing; doesn't work on Pyston yet
Copy over some code from CPython
Basic functionality working, with warnings
Change our type creation to add the extra PyHeapTypeObject fields

This is so that we can add the CAPI slots for things like tp_as_sequence,
so that C extensions will work properly.

I don't think there are very many extensions that would do this but let's
still support it for now; it further bloats the type objects but only as
much as it does in CPython as well.
Import more code from CPython, and add the slotdef for mp_subscript
Add mapping methods and corresponding test
Add the rest of the sequence methods
Merge branch 'getitem'