Refactor how __dict__ and __weakref__ interact with __slots__.

Refactor how __dict__ and __weakref__ interact with __slots__.

1. You can now have __dict__ and/or __weakref__ in your __slots__
   (before only __weakref__ was supported).  This is treated
   differently than before: it merely sets a flag that the object
   should support the corresponding magic.

2. Dynamic types now always have descriptors __dict__ and __weakref__
   thrust upon them.  If the type in fact does not support one or the
   other, that descriptor's __get__ method will raise AttributeError.

3. (This is the reason for all this; it fixes SF bug 575229, reported
   by Cesar Douady.)  Given this code:
      class A(object): __slots__ = []
      class B(object): pass
      class C(A, B): __slots__ = []
   the class object for C was broken; its size was less than that of
   B, and some descriptors on B could cause a segfault.  C now
   correctly inherits __weakrefs__ and __dict__ from B, even though A
   is the "primary" base (C.__base__ is A).

4. Some code cleanup, and a few comments added.