Add `struct FileTimes` to contain the relevant file timestamps, since
most platforms require setting all of them at once. (This also allows
for future platform-specific extensions such as setting creation time.)

Add `File::set_file_time` to set the timestamps for a `File`.

Implement the `sys` backends for UNIX, macOS (which needs to fall back
to `futimes` before macOS 10.13 because it lacks `futimens`), Windows,
and WASI.

This would otherwise silently ignore the attempt, since 0 serves as a
flag to not set a timestamp.

futimens requires Android API level 19, and std still supports older API
levels.

…n futimens

Just return `io::ErrorKind::Unsupported` instead.

Redox doesn't appear to support `UTIME_OMIT`, so we can't set file times
individually.