Some long support, trying to add tp_as_buffer
Implement the sequence iterator protocol

ie if you just have a __getitem__ method, you are still iterable.

This patch is unfortunately yet another performance regression, since
we obscure the behavior for known types (ie no longer try to go directly
to the __iter__).  Should make this into a call into type-specific behavior,
so that known types can use __iter__ if they have one?