Microptimize MoveList loop

Add MOVE_NONE at the tail, this allows to loop
across MoveList checking for *it != MOVE_NONE,
and because *it is used imediately after compiler
is able to reuse it.

With this small patch perft speed increased of 3%

And it is also a semplification !

No functional change.

src/search.cpp

@@ -163,7 +163,7 @@ size_t Search::perft(Position& pos, Depth depth) {
   size_t cnt = 0;
   CheckInfo ci(pos);
 
-  for (MoveList<LEGAL> it(pos); !it.end(); ++it)
+  for (MoveList<LEGAL> it(pos); *it; ++it)
   {
       pos.do_move(*it, st, ci, pos.move_gives_check(*it, ci));
       cnt += perft(pos, depth - ONE_PLY);