Return iterators instead of collected vecs

- Change the return type of metadata convenience iter functions to
  actual iterators instead of allocated collections
- The iterator item type is set to be a reference

examples/playlist_tracks.rs

@@ -36,7 +36,7 @@ async fn main() {
     let plist = Playlist::get(&session, plist_uri).await.unwrap();
     println!("{:?}", plist);
     for track_id in plist.tracks() {
-        let plist_track = Track::get(&session, track_id).await.unwrap();
+        let plist_track = Track::get(&session, *track_id).await.unwrap();
         println!("track: {} ", plist_track.name);
     }
 }