Cleanup cl-macs namespace. Add macro helpers in macroexp.el.

Cleanup cl-macs namespace.  Add macro helpers in macroexp.el.
* emacs-lisp/macroexp.el (macroexp-progn, macroexp-let*, macroexp-if)
(macroexp-let², macroexp--const-symbol-p, macroexp-const-p)
(macroexp-copyable-p): New functions and macros.
* emacs-lisp/edebug.el (edebug-unwrap):
* emacs-lisp/disass.el (disassemble-internal): Use macroexp-progn.
* emacs-lisp/pcase.el: Use macroexp-let*, macroexp-if, ...
(pcase--let*): Remove.
* emacs-lisp/bytecomp.el (byte-compile-const-symbol-p)
(byte-compile-constp): Remove.  Use macroexp--const-symbol-p and
macroexp-const-p instead.
* emacs-lisp/byte-opt.el: Use macroexp-const-p and macroexp-progn.

* emacs-lisp/cl-macs.el: Clean up the name space by using "cl--"
instead of "cl-" for internal definitions.  Use macroexp-const-p.
(cl-old-bc-file-form): Remove var.
(cl-const-exprs-p): Remove fun.
(cl-labels, cl-macrolet): Use backquote.
(cl-lexical-let): Use cl-symbol-macrolet.  Don't use cl-defun-expander.
(cl-defun-expander, cl-byte-compile-compiler-macro): Remove fun.
(cl-define-setf-expander): Rename from cl-define-setf-method.
* emacs-lisp/cl.el: Adjust alias for define-setf-method.

* international/mule-cmds.el: Don't require CL.
(view-hello-file): Don't use `letf'.