Extraction to OCaml

Hello,

This isn't much, I don't know if it is interesting to you but since I made it, I might as well propose it here.
As the title says, this commit introduce OCaml extraction, since its not too much different from the Haskell one, I don't know if it serves truly a purpose in this repository but well, at least it will be useful to me even if not merged. :)

(and thanks for this useful repository !)
Have a good day !