Add ExpandedSwarm

[tomaka]

Turns Swarm into a type Swarm<...> = ExpandedSwarm<...> where ExpandedSwarm has a lot more template parameters.

Unfortunately Rust still has a lot of limitations when it comes to bounds. I got blocked in Substrate because of some combination of requirements that wasn't possible to satisfy in a struct declaration. This PR makes it possible to use the swarm in ways that weren't possible by removing the mandatory where TBehaviour: NetworkBehaviour requirement from Swarm.

[twittner]

Having similarly named types RawSwarm, ExpandedSwarm and Swarm is certainly not optimal. I wonder if we would be better off without the NetworkBehaviour trait and instead have users consume events from the swarm stream and providing them with a command channel for the swarm?

[tomaka]

better off without the NetworkBehaviour trait and instead have users consume events from the swarm stream and providing them with a command channel for the swarm?

From my understanding that basically means using the RawSwarm directly.
The NetworkBehaviour trait is mostly for composability, so that we can provide behaviours and let the user combine behaviours together in an easy way.