Don't copy the key in do_move

It will be overwritten anyway.

Also other little small touches that seem to increase
speed more then the whole enum Score patch series :-(

Optimization is really a black art.

No functional change.

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

src/position.h

@@ -87,12 +87,13 @@ enum Phase {
 /// must be passed as a parameter.
 
 struct StateInfo {
-  Key key, pawnKey, materialKey;
+  Key pawnKey, materialKey;
   int castleRights, rule50, pliesFromNull;
   Square epSquare;
   Score value;
   Value npMaterial[2];
 
+  Key key;
   PieceType capture;
   Bitboard checkersBB;
   StateInfo* previous;