[board] Add PieceSet::find_pieces

Takes a Piece and returns a BitBoard representing where pieces of that shape and
color are on the board.

board/src/piece_sets.rs

@@ -85,6 +85,12 @@ impl PieceSet {
         self.mailbox.get(square)
     }
 
+    pub(crate) fn find_pieces(&self, piece: Piece) -> BitBoard {
+        let color_occupancy = self.color_occupancy[piece.color as usize];
+        let shape_occupancy = self.shape_occupancy[piece.shape as usize];
+        color_occupancy & shape_occupancy
+    }
+
     pub(crate) fn place(
         &mut self,
         piece: Piece,