[board] Implement Color::other() that swaps the color for the other

board/src/piece.rs

@@ -12,6 +12,13 @@ impl Color {
     pub fn iter() -> impl Iterator<Item = Color> {
         [Color::White, Color::Black].into_iter()
     }
+
+    pub fn other(&self) -> Color {
+        match self {
+            Color::White => Color::Black,
+            Color::Black => Color::White,
+        }
+    }
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]