[board] Add occupancy methods that return BitBoards

board/src/board.rs

@@ -82,7 +82,24 @@ impl Board {
 }
 
 impl Board {
-    #[must_use]
+    pub fn occupancy(&self) -> BitBoard {
+        self.pieces.occpuancy()
+    }
+
+    pub fn vacancy(&self) -> BitBoard {
+        !self.occupancy()
+    }
+
+    pub fn friendly_occupancy(&self, color: Color) -> BitBoard {
+        self.pieces.friendly_occupancy(color)
+    }
+
+    pub fn opposing_occupancy(&self, color: Color) -> BitBoard {
+        self.pieces.opposing_occupancy(color)
+    }
+}
+
+impl Board {
     pub fn display(&self) -> DiagramFormatter<'_> {
         DiagramFormatter::new(self)
     }