Latest rust fmt

Rust nightly no longer likes .to_string methods. I've removed them entirely where possible, and added format! macros other places.

Unfornately SocketAddr does have a fmt method defined at all, so I had to get creative. Ideally, this should be reinstated in rust-lang.

Where slices are required, I've used format! as well, but that feels sloppy and wrong. Let me know if there's a better way to do this. Thanks!