New "object debugger" which checks frequently if all Python objects tracked
by the garbage collector are consistent: gc.enable_object_debugger()
and gc.disable_object_debugger().

* Py_FatalError() and _PyObject_AssertFailed() now disable the GC
  object debugger to prevent reentrant calls.
* Fix _PyObject_Dump() for ob_type=NULL

Disable debugger in PyObject_GC_Del() and _PyObject_GC_Resize(): the
debugger is just too slow.