Weak reference support, closing SF bug #451773.

Classes that don't use __slots__ have a __weakref__ member added in
the same way as __dict__ is added (i.e. only if the base didn't
already have one).  Classes using __slots__ can enable weak
referenceability by adding '__weakref__' to the __slots__ list.

Renamed the __weaklistoffset__ class member to __weakrefoffset__ --
it's not always a list, it seems.  (Is tp_weaklistoffset a historical
misnomer, or do I misunderstand this?)