Add UdpSocket::PeerAddr

[yoshuawuyts]

This was just stabilized in std; we should consider adding it too.

rust-lang/rust#64728

[ansrivas]

@yoshuawuyts I quickly looked in the source code, please correct me if I am wrong, this is going to be currently blocked by mio's new release with peer_addr implementation right?

[yoshuawuyts]

@ansrivas yeah I guess you're right.

[zhaxzhax]

I am a beginner in Rust. And I am really delighted to see there is a issue I can fix. I am gonna take this one.

[akshatagarwl]

@yoshuawuyts It seems like this can be closed now.