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.

protocols/floodsub/src/layer.rs

@@ -329,7 +329,7 @@ impl Floodsub {
 
 impl NetworkBehaviour for Floodsub {
     type ConnectionHandler = OneShotHandler<FloodsubProtocol, FloodsubRpc, InnerMessage>;
-    type OutEvent = FloodsubEvent;
+    type ToSwarm = FloodsubEvent;
 
     fn handle_established_inbound_connection(
         &mut self,
@@ -470,7 +470,7 @@ impl NetworkBehaviour for Floodsub {
         &mut self,
         _: &mut Context<'_>,
         _: &mut impl PollParameters,
-    ) -> Poll<ToSwarm<Self::OutEvent, THandlerInEvent<Self>>> {
+    ) -> Poll<ToSwarm<Self::ToSwarm, THandlerInEvent<Self>>> {
         if let Some(event) = self.events.pop_front() {
             return Poll::Ready(event);
         }