Add a backward-compatible version of seq-reverse

Add a backward-compatible version of seq-reverse

* lisp/emacs-lisp/seq.el (seq-reverse): Add a backward-compatible
version of seq-reverse that works on sequences in Emacs 24.  Bump
version to 1.2.
* test/automated/seq-tests.el (test-seq-reverse, test-seq-group-by):
Add a test for seq-reverse and update test for seq-group-by to test
vectors and strings, not only lists.