use unstable sort, small optimization

src/ai.rs

@@ -93,7 +93,7 @@ pub fn alphabeta(mut game: Game, depth: u8, mut alpha: i8, mut beta: i8) -> (Boa
     // since we're considering the "best" moves first.
     // We do this by mapping moves to ranked moves and then sorting.
     let mut moves = explode_board(moves).map(MoveRank::from).collect::<Vec<_>>();
-    moves.sort();
+    moves.sort_unstable();
     let moves = moves
         .into_iter()
         .map(MoveRank::into_inner)