Refactored some of the Py_TRACE_REFS code. New private API function

Refactored some of the Py_TRACE_REFS code.  New private API function
_Py_AddToAllObjects() that simply inserts an object at the front of
the doubly-linked list of all objects.  Changed PyType_Ready() (the
 closest thing we've got to a choke point for type objects) to call
that.