RFC: Add SocketAddr conversion trait to std::io::net and generalize socket constructors

[Rendered view](https://github.com/netvl/rfcs/blob/master/active/0000-socket-addr-conversion-trait.md)
In that case, I think, it makes sense to add default method to the trait which returns the head of the vector, while this new method returning a vector becomes the abstract one. I'll add this to the RFC.