New rules for deleting modules. Rather than having an elaborate

New rules for deleting modules.  Rather than having an elaborate
scheme based on object's types, have a simple two-phase scheme based
on object's *names*:

	/* To make the execution order of destructors for global
	   objects a bit more predictable, we first zap all objects
	   whose name starts with a single underscore, before we clear
	   the entire dictionary.  We zap them by replacing them with
	   None, rather than deleting them from the dictionary, to
	   avoid rehashing the dictionary (to some extent). */