lisp/emacs-lisp/fcr.el: Make it available to cl-generic

lisp/emacs-lisp/fcr.el: Make it available to cl-generic

* lisp/loadup.el: Load `fcr`.

* lisp/emacs-lisp/fcr.el: Don't use `cl-lib` at runtime.
(fcr--copy): Use `named-let` instead of `cl-mapcar`.
(fcr--struct-tag, fcr--struct-specializers, fcr--struct-generalizer)
(cl-generic-generalizers): Move cl-generic support to cl-generic.

* lisp/emacs-lisp/cl-generic.el (cl--generic-fcr-tag)
(cl-generic--fcr-specializers, cl-generic--fcr-generalizer)
(cl-generic-generalizers): Move FCR support from `fcr.el`.