try_clone not implemented on UdpSocket

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?