[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/position/position.rs

@@ -109,6 +109,17 @@ impl Position {
         true
     }
 
+    pub(crate) fn rook_for_castle(&self, player: Color, castle: Castle) -> Option<PlacedPiece> {
+        let square = match (player, castle) {
+            (Color::White, Castle::KingSide) => Square::H1,
+            (Color::White, Castle::QueenSide) => Square::A1,
+            (Color::Black, Castle::KingSide) => Square::H8,
+            (Color::Black, Castle::QueenSide) => Square::A8,
+        };
+
+        self.piece_on_square(square)
+    }
+
     pub fn moves(&self) -> Moves {
         Moves::new(self, self.color_to_move)
     }