global: Py_MakePendingCalls.  Also guard against recursive calls

	* Include/classobject.h, Objects/classobject.c,
	Python/{ceval.c,bltinmodule.c}: entirely redone operator
	overloading.  The rules for class instances are now much more
	relaxed than for other built-in types
	(whose coerce must still return two objects of the same type)