Networking API UDP peer_addr Missing

[pinkforest]

Whilst working on lunatic-solutions/lunatic-rs#17 at lunatic-solutions/lunatic-rs#33
I realised that we don't have peer_addr() in host when we connect() and whilst using recv() and send()
If I would cache it in UdpSocket via connect() that would make UdpSocket mutable on connect() which doesn't match std's

[bkolobara]

I think we should add it. At the same time we could add peer_addr for TcpStream too.