feat(swarm): rename associated types for message passing

Previously, the associated types on `NetworkBehaviour` and `ConnectionHandler` carried generic names like `InEvent` and `OutEvent`. These names are _correct_ in that `OutEvent`s are passed out and `InEvent`s are passed in but they don't help users understand how these types are used.

In theory, a `ConnectionHandler` could be used separately from `NetworkBehaviour`s but that is highly unlikely. Thus, we rename these associated types to indicate, where the message is going to be sent to:

- `NetworkBehaviour::OutEvent` is renamed to `ToSwarm`: It describes the message(s) a `NetworkBehaviour` can emit to the `Swarm`. The user is going to receive those in `SwarmEvent::Behaviour`.
- `ConnectionHandler::InEvent` is renamed to `FromBehaviour`: It describes the message(s) a `ConnectionHandler` can receive from its behaviour via `ConnectionHandler::on_swarm_event`. The `NetworkBehaviour` can send it via the `ToSwarm::NotifyHandler` command.
- `ConnectionHandler::OutEvent` is renamed to `ToBehaviour`: It describes the message(s) a `ConnectionHandler` can send back to the behaviour via the now also renamed `ConnectionHandlerEvent::NotifyBehaviour` (previously `ConnectionHandlerEvent::Custom`)

Resolves: libp2p#2854.

Pull-Request: libp2p#3848.

src/behaviour.rs

@@ -209,7 +209,7 @@ pub struct Behaviour {
 
     last_probe: Option<Instant>,
 
-    pending_actions: VecDeque<ToSwarm<<Self as NetworkBehaviour>::OutEvent, THandlerInEvent<Self>>>,
+    pending_actions: VecDeque<ToSwarm<<Self as NetworkBehaviour>::ToSwarm, THandlerInEvent<Self>>>,
 
     probe_id: ProbeId,
 
@@ -427,7 +427,7 @@ impl Behaviour {
 impl NetworkBehaviour for Behaviour {
     type ConnectionHandler =
         <request_response::Behaviour<AutoNatCodec> as NetworkBehaviour>::ConnectionHandler;
-    type OutEvent = Event;
+    type ToSwarm = Event;
 
     fn poll(&mut self, cx: &mut Context<'_>, params: &mut impl PollParameters) -> Poll<Action> {
         loop {
@@ -594,7 +594,7 @@ impl NetworkBehaviour for Behaviour {
     }
 }
 
-type Action = ToSwarm<<Behaviour as NetworkBehaviour>::OutEvent, THandlerInEvent<Behaviour>>;
+type Action = ToSwarm<<Behaviour as NetworkBehaviour>::ToSwarm, THandlerInEvent<Behaviour>>;
 
 // Trait implemented for `AsClient` and `AsServer` to handle events from the inner [`request_response::Behaviour`] Protocol.
 trait HandleInnerEvent {