* lisp/emacs-lisp/rx.el: Make it a superset of sregex.

* lisp/emacs-lisp/rx.el: Make it a superset of sregex.
(rx-constituents): Add `any => "."', mark `repeat' as taking any number
of args, add `regex' alias.
(rx-info): Add arg to distinguish head and standalone forms.
(rx-check, rx-form): Pass the corresponding arg.
(rx-**): Simplify.
(rx-repeat): Make it work for any number of args.
(rx-syntax): Make it accept syntax chars as is.
* lisp/obsolete/sregex.el: Move from emacs-lisp/.
* lisp/emacs-lisp/re-builder.el: Remove sregex support.
* lisp/emacs-lisp/edebug.el (sregexq, rx): Remove redundant defs.