Generalize TPeerId into TConnInfo

[tomaka]

This is the first part of #1030

At the moment, the structs in the nodes module have a TPeerId parameter that notably requires Eq + Hash.

This PR turns TPeerId into TConnInfo, and introduces a new trait that TConnInfo must implement:

/// Information about a connection.
pub trait ConnectionInfo {
    /// Identity of the node we are connected to.
    type PeerId: Eq + Hash;

    /// Returns the identity of the node we are connected to on this connection.
    fn peer_id(&self) -> &Self::PeerId;
}

impl ConnectionInfo for PeerId {
    type PeerId = PeerId;

    fn peer_id(&self) -> &PeerId {
        self
    }
}

By default, a TConnInfo is a PeerId. But after this PR, it can consist of anything that contains a PeerId.

[twittner]

How about removing peer_id()?

[tomaka]

It is there for convenience and is used a lot in raw_swarm. Can I keep it? 🙏

[twittner]

Of course you can keep it 😄. It was just a thought.