Restore ssrmatching and its reverse dependencies

4181269f removed ssrmatching and
everything that needed it because upstream had shipped a couple of
files with bad license headers. Those files have now been fixed
(https://github.com/coq/coq/pull/9282), so grab them from master and
apply them in a patch. This restores ssrmatching to the Coq standard
library.

Once upstream cuts its next release, we should be able to delete the
patch and simply import the files from the upstream tarball.