try_clone not implemented on UdpSocket

[juerkkil]

Noticed that async_std::net::UdpSocket does not implement try_clone() function, which is implemented in standard lib.

Tested out briefly on a local env and it seems to me that something like this would do the trick:

    pub fn try_clone(&self) -> UdpSocket {
        UdpSocket::from(self.watcher.get_ref().try_clone().expect("oops"))
    }

Is it a good idea to craft a PR, or is there some particular reason why this is not implemented?