(cl--generic-compiler): Revert last change

(cl--generic-compiler): Revert last change

That change (introduced to circumvent an error now that `seq.el` is
preloaded) caused all dispatchers to be left uncompiled, which slows down
method dispatch very significantly.  Fix the problem in the old way,
i.e. by adding an explicit call to `cl--generic-prefill-dispatchers`.

* lisp/emacs-lisp/cl-generic.el (cl--generic-compiler): Revert last change.
Add (cl--generic-prefill-dispatchers 1 integer) instead to handle
the new dispatchers needed for `seq.el`.
(cl--generic-prefill-generalizer-sample): New function.
(cl--generic-get-dispatcher): Use it to signal an error giving precise
instructions for what to do if we're about the load the byte-compiler
during the preload.
(cl--generic-oclosure-generalizer): Rename from
`cl-generic--oclosure-generalizer` for consistency with all
other generalizers.