Move gensym to core Elisp

Move gensym to core Elisp

* doc/lispref/symbols.texi (Creating Symbols): Mention gensym right
after make-symbol.
* etc/NEWS: Mention.
* lisp/emacs-lisp/cl-macs.el (cl--gensym-counter): Alias to
gensym-counter.
(cl-gensym): Alias to gensym.
* lisp/emacs-lisp/cl.el: Remove gensym from list of aliases.
* lisp/emacs-lisp/edebug.el (edebug-make-enter-wrapper):
* lisp/emacs-lisp/ert-x.el (ert-with-message-capture):
(ert--expand-should-1, ert--expand-should):
(ert--should-error-handle-error):
* lisp/emacs-lisp/generator.el (cps--gensym):
* lisp/emacs-lisp/gv.el (setf):
* lisp/emacs-lisp/inline.el (inline--do-letlisteval):
* lisp/emacs-lisp/pcase.el (pcase--make-docstring, pcase-dolist):
(pcase--funcall, pcase--u1): Use gensym.
* lisp/subr.el (gensym-counter): New variable.
(gensym): New function, assimilated from cl-lib.