Characterize functions in terms of type specifiers

Characterize functions in terms of type specifiers

	* lisp/emacs-lisp/comp.el (comp-known-type-specifiers): New const
	in place of `comp-known-ret-types' and `comp-known-ret-ranges'.
	(comp-constraint): New struct to separate the constraint side of
	an mvar.
	(comp-constraint-f): Analogous for functions.
	(comp-mvar): Rework and include `comp-constraint'.
	(comp-type-spec-to-constraint): New function.
	(comp-known-constraints-h): New const.
	(comp-func-ret-typeset, comp-func-ret-range): Rework.
	(comp-fwprop-insn): Fix.
	* test/src/comp-tests.el (destructure-type-spec): New testcase.