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: #2854.

Pull-Request: #3848.

swarm/src/handler/select.rs

@@ -179,8 +179,8 @@ where
     TProto1: ConnectionHandler,
     TProto2: ConnectionHandler,
 {
-    type InEvent = Either<TProto1::InEvent, TProto2::InEvent>;
-    type OutEvent = Either<TProto1::OutEvent, TProto2::OutEvent>;
+    type FromBehaviour = Either<TProto1::FromBehaviour, TProto2::FromBehaviour>;
+    type ToBehaviour = Either<TProto1::ToBehaviour, TProto2::ToBehaviour>;
     type Error = Either<TProto1::Error, TProto2::Error>;
     type InboundProtocol = SelectUpgrade<
         SendWrapper<<TProto1 as ConnectionHandler>::InboundProtocol>,
@@ -201,7 +201,7 @@ where
         SubstreamProtocol::new(choice, (i1, i2)).with_timeout(timeout)
     }
 
-    fn on_behaviour_event(&mut self, event: Self::InEvent) {
+    fn on_behaviour_event(&mut self, event: Self::FromBehaviour) {
         match event {
             Either::Left(event) => self.proto1.on_behaviour_event(event),
             Either::Right(event) => self.proto2.on_behaviour_event(event),
@@ -222,7 +222,7 @@ where
         ConnectionHandlerEvent<
             Self::OutboundProtocol,
             Self::OutboundOpenInfo,
-            Self::OutEvent,
+            Self::ToBehaviour,
             Self::Error,
         >,
     > {