[board] Implement PieceSet::mailbox() method to return a reference to its Mailbox

board/src/piece_sets.rs

@@ -30,9 +30,9 @@ impl Default for PlacePieceStrategy {
 /// placement of pieces on the board.
 #[derive(Clone, Debug, Default, Eq, Hash, PartialEq)]
 pub(crate) struct PieceSet {
+    mailbox: Mailbox,
     by_color: ByColor,
     by_color_and_shape: ByColorAndShape,
-    mailbox: Mailbox,
 }
 
 impl PieceSet {
@@ -65,6 +65,10 @@ impl PieceSet {
         }
     }
 
+    pub(crate) fn mailbox(&self) -> &Mailbox {
+        &self.mailbox
+    }
+
     /// A [`BitBoard`] representing all the pieces currently on the board. Other
     /// engines might refer to this concept as 'occupancy'.
     pub(crate) fn all_pieces(&self) -> BitBoard {