lwt_unix: sub-second precision for stat results

This patch does make lwt_Unix.stat results unequal to the corresponding Unix.stat results, since Unix.stat still only uses the second-level fields. I'd like to fix that as well, but forking and compiling ocaml itself is harder than lwt.

(there's an open bug if anyone else wants to jump in: http://caml.inria.fr/mantis/view.php?id=6285)

**note**: The code is copied from Jane street core (https://github.com/janestreet/core/blob/master/lib/unix_stubs.c#L426). The code is under the APL, although I don't know if copyright attribution is required, since the code is driven almost entirely by maths and system APIs.