simplify FutureMoves::rebuild_from_board

src/logic/future_moves.rs

@@ -389,9 +389,7 @@ impl FutureMoves {
     }
 
     fn rebuild_from_board(&mut self, board: Board) {
-        self.arena.clear();
-        self.arena
-            .push(Move::new(None, board, !self.agent_color, self.agent_color));
+        self.arena = vec![Move::new(None, board, !self.agent_color, self.agent_color)];
         self.current_root = Some(0);
         self.current_depth = 0;
         self.board = board;