Make seq.el more extensible by using cl-defmethod

Make seq.el more extensible by using cl-defmethod

* lisp/emacs-lisp/seq.el: Define seq.el functions using cl-defmethod to
make it easier to extend seq.el with new "seq types".
* test/automated/seq-tests.el (test-setf-seq-elt): New test.
* lisp/emacs-lisp/cl-extra.el (cl-subseq): Move back the definition of
subseq in cl-extra.el, and use it in seq.el.