[board] Implement position::builders::MoveBuilder

This builder takes a Position and a Move, validates the move, and makes the move
in that position. Its build() method returns a new Position with the move made.

board/src/bitboard/bitboard.rs

@@ -61,7 +61,7 @@ impl BitBoard {
         *self |= sq_bb
     }
 
-    fn clear_square(&mut self, sq: Square) {
+    pub fn clear_square(&mut self, sq: Square) {
         let sq_bb: BitBoard = sq.into();
         *self &= !sq_bb
     }