Implement AsRawFd for &Pty

[crumblingstatue]

Some APIs, like nonblock::NonBlockingReader::from_fd take a value that implements AsRawFd.

Pty implements AsRawFd, but if we pass it by value, it will get moved, as it doesn't implement Clone or Copy.

With this change, &Pty can now be passed without moving the Pty.

[doy]

this feels a bit odd - other things that implement AsRawFd don't do this, so it feels like it would make more sense to change this on the nonblock side? i'm open to arguments for why that shouldn't be the case, but forcing every crate to implement both versions seems weird to me.

[crumblingstatue]

I can try opening a pull request on the nonblock repo later today, and see if there are any objections.

[crumblingstatue]

Well, I tried to see if I can make a simple change on the nonblock-rs side, but I'm not sure what to do.
The way it works is by taking ownership of the reader

pub struct NonBlockingReader<R: AsRawFd + Read> {
    eof: bool,
    reader: R,
}

I'm not sure how to make that work with references.
Let me know if you have any ideas, but to me, it seems to me this would be a simple addition to this crate, while I would have to make intrusive changes to nonblock-rs.