Implemented os_utime

To replace the code in fileio.c.
Fun fact: libuv does not use utimes yet (if available)
Since this function is only use by file info structs, I think it would make more sense to write `os_fileinfo_utime()`. Otherwise, I'd have to ask why it takes `uv_timespec_t`s when it obviously needs doubles, and the relation between this and `uv_stat` isn't clear.