core/src/transport: Poll Transport directly, remove Transport::Listener (#2652)

Remove the concept of individual `Transport::Listener` streams from `Transport`.
Instead the `Transport` is polled directly via `Transport::poll`. The
`Transport` is now responsible for driving its listeners.

swarm/src/test.rs

@@ -23,9 +23,7 @@ use crate::{
     PollParameters,
 };
 use libp2p_core::{
-    connection::{ConnectionId, ListenerId},
-    multiaddr::Multiaddr,
-    ConnectedPoint, PeerId,
+    connection::ConnectionId, multiaddr::Multiaddr, transport::ListenerId, ConnectedPoint, PeerId,
 };
 use std::collections::HashMap;
 use std::task::{Context, Poll};