Edebug: Make it possible to debug `gv-expander`s in `declare`

Edebug: Make it possible to debug `gv-expander`s in `declare`

Arrange for declarations to be able to specify their own specs via
the `edebug-declaration-spec` property.

* lisp/emacs-lisp/edebug.el: (edebug--get-declare-spec): New function.
(def-declarations): New spec element.
(defun, defmacro): Use it in their spec.

* lisp/emacs-lisp/gv.el (gv-expander, gv-setter):
Set `edebug-declaration-spec`.

* test/lisp/emacs-lisp/edebug-tests.el (edebug-tests-gv-expander): New test.

* test/lisp/emacs-lisp/edebug-resources/edebug-test-code.el
(edebug-test-code-use-gv-expander): New test case.