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.
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