IterBytes for IpAddr and SocketAddr
auto merge of #12149 : thomaslee/rust/ipaddr_deriving_iter_bytes, r=cmr

This is a fairly trivial (but IMHO handy) change to implement IterBytes for IpAddr and SocketAddr.

I originally stumbled across this because I wanted to use a SocketAddr as a HashMap key and discovered that I couldn't do it directly. Had to impl IterBytes on a new intermediate type to work around it.