'cl-gensym' was simply moved here, but let us take an opportunity to
shed some historical baggage.
* lisp/subr.el (gensym): Remove special treatment of PREFIX as a
number. Use "g" as prefix to differentiate from cl-gensym defaults.
* doc/lispref/symbols.texi (Creating Symbols): Update accordingly.
* lisp/emacs-lisp/cl-macs.el (cl--gensym-counter, cl-gensym): Restore.