[board] Fix a bug in PieceSet::opposing_occupancy

This method should compute occupancy of everything except the provided color. Instead
it was computing the inverse.

board/src/piece_sets.rs

@@ -69,13 +69,15 @@ impl PieceSet {
     }
 
     pub fn opposing_occupancy(&self, color: Color) -> BitBoard {
+        let color_index = color as usize;
+
         self.color_occupancy.iter().enumerate().fold(
             BitBoard::default(),
             |acc, (index, occupancy)| {
-                if index == color as usize {
-                    acc | occupancy
-                } else {
+                if index == color_index {
                     acc
+                } else {
+                    acc | occupancy
                 }
             },
         )