[draft] Add borrowing for fs_utf8::Dir

[cgwalters]

I'd like to start using fs_utf8::Dir, but not in a "big bang" fashion where I wholesale convert the entire codebase. It feels normal and natural to have a non-utf8 Dir instance and maybe sometimes just want to "view" it as utf8.

At an efficiency level, having to create a new owned fd for this feels unnecessary.

This sketches out an implementation where we can also support borrowing an existing fd.

This doesn't compile, but before I do a lot more typing on this - consider it an RFC. Is this a good direction?

[cgwalters]

Oh yeah, I guess things like

#[cfg(not(windows))]
impl From<Dir> for OwnedFd {
    #[inline]
    fn from(dir: Dir) -> OwnedFd {
        dir.cap_std.into()
    }
}

can't work anymore...ugh, would need a new BorrowedDir or something.

[cgwalters]

OK, trying #362 instead