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

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

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

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

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