Avoid some OCaml 4.02 warnings

There is still some warnings for which I don't know what to do:
 * ```String.create```: I don't know how to replace it with ```Buffer.create```. It uses ```input``` and ```output``` with it and I don't know how to replace those functions with functions that works with Buffers.
 * ```Filename.temp_dir_name```: the non-deprecated function (```Filename.get_temp_dir_name```) is only available since OCaml 4.00 and I don't know if we rely on a lower version of OCaml.

PS: Litmus doesn't compile because of a missing ```Topology``` module.