Move EIEIO's C3 linearization code to `subr.el`

Move EIEIO's C3 linearization code to `subr.el`

The code was used to linearize the EIEIO class hierarchy, since
it results in saner results than things like BFS or DFS.
By moving it to `subr.el` we get to benefit from that same
advantage both in `cl--class-allparents` and
in `derived-mode-all-parents`.

* lisp/subr.el (merge-ordered-lists): New function.
(derived-mode-all-parents): Use it to improve parent ordering.

* lisp/emacs-lisp/eieio-core.el (eieio--c3-candidate)
(eieio--c3-merge-lists): Delete functions, replaced by
`merge-ordered-lists`.
(eieio--class-precedence-c3): Use `merge-ordered-lists`.

* lisp/emacs-lisp/cl-preloaded.el (cl--class-allparents):
Use `merge-ordered-lists` to improve parent ordering.
* lisp/emacs-lisp/cl-macs.el (cl--struct-all-parents): Delete function.
(cl--pcase-mutually-exclusive-p): Use `cl--class-allparents` instead.