Implement a reference queue API.

	* gc.c: A reference queue allows one to queue
	callbcks for when objects are collected.
	It allows for safe cleanup of objects that can
	only be done when it is effectively collected.
	The major difference with regular finalization
	is that the collector makes sure the object
	was collected - and can't be resurrected.

	* gc-internal.h: Export entrypoints for the
	new API.