Further smplify pawn endgames

Dumb down a bit the code and trade some possible
speed (but this is far from hot path anyhow) for
some added readability for the layman.

No functional change.

src/endgame.h

@@ -91,7 +91,7 @@ struct Endgame : public EndgameBase<T> {
   T operator()(const Position&) const;
 
 private:
-  Color strongSide, weakSide;
+  const Color strongSide, weakSide;
 };