Split variable macro env from function env

Split variable macro env from function env

* lisp/emacs-lisp/cl-macs.el (cl--sm-macroexpand): Remove.
(cl-symbol-macrolet): Instead of adding each binding directly into the
main environment with a special key format, put all symbol macro
bindings into a single entry in the main environment under
`:cl-symbol-macros'.
(cl--sm-macroexpand): Look up symbol bindings in the
`:cl-symbol-macros' entry of the environment.