Have Transport::Listeners produce ListenerEvents. (#1032)

Replace the listener and address pair returned from `Transport::listen_on` with just a listener that produces `ListenerEvent` values which include upgrades as well as address changes.

core/src/lib.rs

@@ -106,3 +106,23 @@ impl std::ops::Not for Endpoint {
     }
 }
 
+impl Endpoint {
+    /// Is this endpoint a dialer?
+    pub fn is_dialer(self) -> bool {
+        if let Endpoint::Dialer = self {
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Is this endpoint a listener?
+    pub fn is_listener(self) -> bool {
+        if let Endpoint::Listener = self {
+            true
+        } else {
+            false
+        }
+    }
+}
+