* Introduce latches

* Introduce latches

Define a new kind of basic block 'latch' to close over loops.  Its
purpose is for now to emit calls to `comp-maybe-gc-or-quit' but in
future will be usefull for the loop optimizer to exploit unboxes.

	* lisp/emacs-lisp/comp.el (comp-block): New base class.
	(comp-block-lap): New class for LAP derived basic blocks.
	(comp-latch): New class.
	(comp-bb-maybe-add, comp-make-curr-block, comp-emit-handler)
	(comp-emit-switch, comp-emit-switch, comp-limplify-top-level)
	(comp-addr-to-bb-name, comp-limplify-block)
	(comp-limplify-function): Update logic for new bb objects
	arrangment.
	(comp-latch-make-fill): New function.
	(comp-emit-uncond-jump, comp-emit-cond-jump): Update to emit
	latches.
	(comp-new-block-sym): Add a postfix paramenter.