#4986. Add two common operations: List.sort_uniq (more efficient that sorting then removing duplicates) and Set.of_list (more efficient than folding Set.add over the list).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13876 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02