[feature wish] do not depend on unix in the core package

[Armael]

Using memgraph with js_of_ocaml currently raises a few warnings because of the dependency on unix (for Dot.to_file). It would be slightly cleaner to avoid depending on unix in the core library, and expose file-related helpers in a separate package.

[Gbury]

That's a good point, I'll try and split the Unix-related things into a separate package when i have the time to do so.