Implemented find_opt in Map.S interface

[nicoTolly]

Related to this issue : #940

This Pull request adds a find_opt function to the Map.S interface. It appears that there was already a find_option implemented in Map.Concrete so I just used that. Another implementation was needed in BatSplay.ml so I added one. Also note that a function with the same type is implemented in Map.S.Exceptionless. Maybe we should do some factorization, but for now I changed the code as little as possible.
There was a discussion on the issue on which syntax should be used (about whether or not use the match... exception syntax). I'm still not clear on the subject, so I'm waiting for your comments.

[UnixJunkie]

Thanks a lot for sending a PR.

Can you replace the implementation by this:

let find_opt k m =
  try Some (find k m)
  with Not_found -> None

Because it will compile whatever the OCaml version.

Also, the unit tests are usually a few line as comments in the same file.
You don't need to change test_map.ml.

[nicoTolly]

Done. As an aside, it seems that make test does not automatically recompile every changed files (at some point, make test failed, but make clean/make test succeeded without changing the code). Is it a known problem ?

[UnixJunkie]

Thanks a lot, perfect!

[UnixJunkie]

I think I noticed this recompile problem. But, I am too lazy to dig into a hairy Makefile.
Also, we should use dune and get rid of the current build system in the long run, which
also adds to my low motivation...

[rixed]

FTR: these days dependencies are not handled with make but with ocamlbuild

[rixed]

find_opt has been added only in v4.05 of ocaml stdlib whereas batteries claim to support ocaml >= 4.00, therefore compilation is broken for those older compiler.
Anyway, the idea of the Ord module is to use the provided comparison function though, so Stdlib.find_opt won't cut it.

Fixed in v3.0.0 I think.