Add support for weak references to code objects. This will be used by an optimization in the incoming Python 3 JIT.

Patch by Reid Kleckner!
diff --git a/Include/code.h b/Include/code.h
index 260c5f0..38b2958 100644
--- a/Include/code.h
+++ b/Include/code.h
@@ -26,6 +26,7 @@
     PyObject *co_lnotab;	/* string (encoding addr<->lineno mapping) See
 				   Objects/lnotab_notes.txt for details. */
     void *co_zombieframe;     /* for optimization only (see frameobject.c) */
+    PyObject *co_weakreflist;   /* to support weakrefs to code objects */
 } PyCodeObject;
 
 /* Masks for co_flags above */