Add a custom derive for NetworkBehaviour

[tomaka]

Adds the possibility to do this:

#[derive(NetworkBehaviour)]
struct MyBehaviour<TSubstream> {
    ping: PingListenerBehaviour<TSubstream>,
    identify: PeriodicIdentifyBehaviour<TSubstream>,
    floodsub: FloodsubBehaviour<TSubstream>,
}

And this automatically implements NetworkBehaviour on MyBehaviour.

Note that the events generated by the fields are ignored by default. To handle them, the user can do this:

#[derive(NetworkBehaviour)]
struct MyBehaviour<TSubstream> {
    ping: PingListenerBehaviour<TSubstream>,
    identify: PeriodicIdentifyBehaviour<TSubstream>,
    #[behaviour(handler = "foo")]
    floodsub: FloodsubBehaviour<TSubstream>,
}

impl<TSubstream> MyBehaviour<TSubstream> {
    // Called whenever the floodsub layer generates a message.
    fn foo(&mut self, event: FloodsubMessage) {
        // ...
    }
}

The one thing that's missing is the situation where the user wants to generate events from their behaviour. This needs thoughts as to how to do it.

[twittner]

Impressive!