New convenience functions in seq.el

New convenience functions in seq.el

Functions to access the first or all but the first elements of
sequences have been repeatedly asked for (the last occurrence being
https://github.com/NicolasPetton/seq.el/issues/9).

* lisp/emacs-lisp/seq.el (seq-first, seq-rest): New functions.
* test/lisp/emacs-lisp/seq-tests.el (test-seq-first, test-seq-rest):
New tests for seq-first and seq-rest.