OCaml Unix fully wrapped.

[briangmilnes]

FST and ML for unix. Added a FDString for string files (useful in test). Added a printable class for unix. Made printable float and double print as #float and #double as we get them Unix. Added printable array and char_code similarly. FStar_Wrap_OCaml.fst/fsti have comments on why they are setup with functions and not just combinators: they reuse better this way but they are not code beautiful.