Gateway: Simplify return value of join/join_gateway (#157)

Replaces the annoying dual-return (i.e., created `Call` *and* `Result<x>`) with a single `Return<Call/ConnectionInfo>`. Users are now informed via that a `Call` is created -- thus, cleanup in event of connection failure is now their responsibility.

Tested using `cargo make ready`.

Closes #65.

examples/serenity/voice_cached_audio/src/main.rs

@@ -233,11 +233,9 @@ async fn join(ctx: &Context, msg: &Message) -> CommandResult {
         .expect("Songbird Voice client placed in at initialisation.")
         .clone();
 
-    let (handler_lock, success_reader) = manager.join(guild_id, connect_to).await;
+    if let Ok(handler_lock) = manager.join(guild_id, connect_to).await {
+        let call_lock_for_evt = Arc::downgrade(&handler_lock);
 
-    let call_lock_for_evt = Arc::downgrade(&handler_lock);
-
-    if let Ok(_reader) = success_reader {
         let mut handler = handler_lock.lock().await;
         check_msg(
             msg.channel_id