Introduce operator~(Piece c)

Small syntactic sugar to reverse piece color.

No functional change.

src/types.h

@@ -359,6 +359,10 @@ inline Square operator~(Square s) {
   return Square(s ^ 56); // Vertical flip SQ_A1 -> SQ_A8
 }
 
+inline Piece operator~(Piece c) {
+  return Piece(c ^ 8);
+}
+
 inline Square operator|(File f, Rank r) {
   return Square((r << 3) | f);
 }