refactor: expose and use THandlerOutEvent type alias (#3368)

Previously, we used the full reference to the `OutEvent` of the `ConnectionHandler` in all implementations of `NetworkBehaviour`. Not only is this very verbose, it is also more brittle to changes. With the current implementation plan for #2824, we will be removing the `IntoConnectionHandler` abstraction. Using a type-alias to refer to the `OutEvent` makes the migration much easier.

swarm/src/test.rs

@@ -24,7 +24,7 @@ use crate::behaviour::{
 };
 use crate::{
     ConnectionHandler, ConnectionId, IntoConnectionHandler, NetworkBehaviour,
-    NetworkBehaviourAction, PollParameters,
+    NetworkBehaviourAction, PollParameters, THandlerOutEvent,
 };
 use libp2p_core::{multiaddr::Multiaddr, transport::ListenerId, ConnectedPoint, PeerId};
 use std::collections::HashMap;
@@ -107,8 +107,7 @@ where
         &mut self,
         _peer_id: PeerId,
         _connection_id: ConnectionId,
-        _event: <<Self::ConnectionHandler as IntoConnectionHandler>::Handler as
-            ConnectionHandler>::OutEvent,
+        _event: THandlerOutEvent<Self>,
     ) {
     }
 }
@@ -455,7 +454,7 @@ where
         &mut self,
         p: PeerId,
         c: ConnectionId,
-        e: <<Self::ConnectionHandler as IntoConnectionHandler>::Handler as ConnectionHandler>::OutEvent,
+        e: THandlerOutEvent<Self>,
     ) {
         assert!(
             self.on_connection_established