[board] Implement PieceSet::iter() as an iterator over its Mailbox

board/src/piece_sets.rs

@@ -75,6 +75,10 @@ impl PieceSet {
         self.by_color.bitboard(color)
     }
 
+    pub(crate) fn iter(&self) -> impl Iterator<Item = PlacedPiece> {
+        self.mailbox.iter()
+    }
+
     pub(super) fn bitboard_for_color(&self, color: Color) -> BitBoard {
         self.by_color.bitboard(color)
     }