Merge short-paths into a fresh branch of trunk.
Also fix a bit unification and subtyping errors.
You now need the flag -short-path to activate short paths.
git-svn-id: http://caml.inria.fr/svn/ocaml/branches/short-paths-4.01@13285 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02