Board: make starting_pos a const variable instead

benches/future_children.rs

@@ -16,7 +16,7 @@ fn extend_layers_no_pruning(depth: usize) -> usize {
         children_eval_method: ChildrenEvalMethod::AverageDivDepth,
     };
     let mut fut = FutureMoves::new(Piece::Black, config);
-    fut.update_from_board(&Board::new().starting_pos());
+    fut.update_from_board(&Board::STARTING_POSITION);
     fut.extend_layers();
     fut.arena_len()
 }