Implement filter_map for OCaml 4.11

[rwmjones]

Downstream for Fedora I wrote this patch which adds supports for Map.filter_map. It is required for OCaml 4.11.
https://src.fedoraproject.org/rpms/ocaml-ptmap/blob/master/f/ptmap-ocaml-4.11.patch

[backtracking]

Thanks for the patch. Just added (commit ea7e437)