Micro optimize copy of new state in do_move()

Instead of copying all, copy only the fields that
are updated incrementally, not the ones that are
recalcuated form scratch anyway.

This reduces copy overhead of 30%.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/position.h

@@ -79,14 +79,15 @@ enum CastleRights {
 /// must be passed as a parameter.
 
 struct StateInfo {
-  Bitboard checkersBB;
   Key key, pawnKey, materialKey;
   int castleRights, rule50;
   Square epSquare;
   Value mgValue, egValue;
+
   PieceType capture;
-  StateInfo* previous;
+  Bitboard checkersBB;
   Move lastMove;
+  StateInfo* previous;
 };