Implemented os_utime

To replace the code in fileio.c.
Fun fact: libuv does not use utimes yet (if available)
> 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.

It's pretty tailored to that specific callsite though. What I mean is: suppose that you want to set the mtime and atime of a specific file with `os_filetime_utime()`. Then you would first need to construct a `FileInfo` struct (can a user do that by himself?) and fill in the just the `stat.st_atim` and `stat.st_mtim` fields, leaving the rest untouched. Said in another way: one is passing way too much info.

I understand and appreciate the simplicity of the `os_fileinfo_utime()` call here though.