[Piece] Implement PieceColor ! operator

.gitignore

@@ -2,3 +2,5 @@
 /build*/
 __pycache__/
 uci-log.txt
+cmake-build-debug
+cmake-build-release

Piece.cpp

@@ -33,6 +33,5 @@ std::ostream& operator<<(std::ostream& os, const Piece& piece) {
 }
 
 PieceColor operator!(PieceColor color) {
-    (void)color;
-    return PieceColor::White;
+    return static_cast<PieceColor>((static_cast<int>(color) + 1) % 2);
 }

Piece.hpp

@@ -5,8 +5,8 @@
 #include <iosfwd>
 
 enum class PieceColor {
-    White,
-    Black
+    White = 0,
+    Black = 1,
 };
 
 enum class PieceType {