Keep track of a type's subclasses (subtypes), in tp_subclasses, which

Keep track of a type's subclasses (subtypes), in tp_subclasses, which
is a list of weak references to types (new-style classes).  Make this
accessible to Python as the function __subclasses__ which returns a
list of types -- we don't want Python programmers to be able to
manipulate the raw list.

In order to make this possible, I also had to add weak reference
support to type objects.

This will eventually be used together with a trap on attribute
assignment for dynamic classes for a major speed-up without losing the
dynamic properties of types: when a __foo__ method is added to a
class, the class and all its subclasses will get an appropriate tp_foo
slot function.