Completely get rid of PyClass and PyInstance.

(classobject.[ch] aren't empty yet because they also define PyMethod.)
This breaks lots of stuff, notably cPickle. But it's a step in the right
direction. I'll clean it up later.
(Also a few unrelated changes, e.g. T_NONE to define a "struct member"
that is always None, and simplification of __hash__ -- these are unfinished.)

Include/Python.h

@@ -7,14 +7,6 @@
 #include "patchlevel.h"
 #include "pyconfig.h"
 
-/* Cyclic gc is always enabled, starting with release 2.3a1.  Supply the
- * old symbol for the benefit of extension modules written before then
- * that may be conditionalizing on it.  The core doesn't use it anymore.
- */
-#ifndef WITH_CYCLE_GC
-#define WITH_CYCLE_GC 1
-#endif
-
 #include <limits.h>
 
 #ifndef UCHAR_MAX