Declaring cl.el obsolete

[Stefan Monnier]

cl-lib was released 6 years and its uptake has been much more successful
than in my wildest dreams.

AFAIK it has been technically obsolete already for the last 6 years, in
that everything it offers is provided by cl-lib or lexical-binding.

I count 6 files in master that still (require 'cl).  Of those, 2 only
require `cl` conditionally (when running on XEmacs or on Emacs<21),
2 are themselves obsolete (cl-compat and lucid.el), so really there are
only 2 packages bundled with Emacs and still actively using `cl`:
vhdl-mode and MH-E.

So I think Emacs-27 is a good time to declare it officially obsolete
(and hence move it to the lisp/obsolete directory).

Comments, objections?


        Stefan