Make 'x in y' and 'x not in y' (PySequence_Contains) play nice w/ iterators.

Make 'x in y' and 'x not in y' (PySequence_Contains) play nice w/ iterators.
NEEDS DOC CHANGES
A few more AttributeErrors turned into TypeErrors, but in test_contains
this time.
The full story for instance objects is pretty much unexplainable, because
instance_contains() tries its own flavor of iteration-based containment
testing first, and PySequence_Contains doesn't get a chance at it unless
instance_contains() blows up.  A consequence is that
    some_complex_number in some_instance
dies with a TypeError unless some_instance.__class__ defines __iter__ but
does not define __getitem__.