tidy up game implementation

src/game.rs

@@ -163,24 +163,14 @@ impl ToString for Game {
     }
 }
 
-pub trait TileIds {
-    fn tile_ids(&self) -> Vec<u64>;
-}
-
-impl TileIds for Game {
+impl Game {
     fn tile_ids(&self) -> Vec<u64> {
         self.tiles.iter().map(|tile| {tile.id}).collect()
     }
-}
 
     /// first available tile ID.
     /// e.g. if current tile IDs are [0, 2, 3] the result will be `1`
     ///      if current tile IDs are [0, 1, 2] the result will be `3`
-pub trait NewTileId {
-    fn new_tile_id(&self) -> u64;
-}
-
-impl NewTileId for Game {
     fn new_tile_id(&self) -> u64 {
         let mut new_id = 0;
 
@@ -197,9 +187,6 @@ impl NewTileId for Game {
 
         new_id + 1
     }
-}
-
-impl Game {
     /// adds a tile safely and returns the new tile ID
     fn add_tile(&mut self, mut tile: Tile) -> u64 {
         let new_id = self.new_tile_id();