Change labels from "l`...'" to "...".

	* src/Scan.hs (isIdentNum): Use isIdent.
	(isIdent): Accept "_" for sake of "print_int".
	(scan): No longer detect "l`...'".
	(scanLabel): Remove.
	* doc/havm.texi: Adjust.

	"size" is a Tiger builtin, but also an Havm keyword for an
	obsolete feature.  Get rid of the latter.

	* src/Ir.hs, src/Annotation.hs, src/Eval.hs, src/High.hs,
	* src/Low.hs, src/Parse.y, src/Preload.hs, src/Print.hs: Label has
	no "size" attribute.
	* src/Code.hs, src/Monad.hs (cframesize): Remove.
	Adjust callers.
	* src/Scan.hs (TokSize): No longer used.