Input: Allow Restartable sources to be lazy

This change is made with queue users in mind. Since sources
of this kind *know* how to (re)create themselves, they can
avoid being created at all until needed.

This also adds machinery to preload tracks *before* they are
needed, for gapless playback on queues and so on. Queues
make use of the event system to do this.

examples/twilight/src/main.rs

@@ -183,7 +183,7 @@ async fn play(msg: Message, state: State) -> Result<(), Box<dyn Error + Send + S
 
     let guild_id = msg.guild_id.unwrap();
 
-    if let Ok(song) = Restartable::ytdl(msg.content.clone()).await {
+    if let Ok(song) = Restartable::ytdl(msg.content.clone(), false).await {
         let input = Input::from(song);
 
         let content = format!(