OClosure: Add support for defmethod dispatch

OClosure: Add support for defmethod dispatch

* lisp/emacs-lisp/oclosure.el (oclosure--class): Add slot `allparents`.
(oclosure--class-make): Add corresponding arg `allparents`.
(oclosure, oclosure--build-class): Pass the new arg to the constructor.
(oclosure--define): Make the predicate function understand subtyping.

* lisp/emacs-lisp/cl-preloaded.el (cl--class-allparents): Move from
`cl-generic.el`.

* lisp/emacs-lisp/cl-generic.el (cl--generic-class-parents): Move to
`cl-preloaded.el` and rename to `cl--class-allparents`.
Adjust all callers.
(cl--generic-oclosure-tag, cl-generic--oclosure-specializers): New functions.
(cl-generic-generalizers) <oclosure-struct>: New generalizer.

* test/lisp/emacs-lisp/oclosure-tests.el (oclosure-test-gen):
New generic function.
(oclosure-test): Add test for dispatch on oclosure types.