Implemented os_utime

To replace the code in fileio.c.
Fun fact: libuv does not use utimes yet (if available)
> [...] os_utime() looks more concise

Except for at the call sites in this PR.

> Personally, I would prefer the following prototype:

I would find that acceptable, especially since one would not need to look up what `uv_timespec_t` means, but still think `os_fileinfo_utime()` could be provided as well, if not *instead* of.