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/position.cpp

@@ -408,7 +408,7 @@ const string Position::pretty(Move move) const {
       ss << square_to_string(pop_lsb(&b)) << " ";
 
   ss << "\nLegal moves: ";
-  for (MoveList<LEGAL> it(*this); !it.end(); ++it)
+  for (MoveList<LEGAL> it(*this); *it; ++it)
       ss << move_to_san(*const_cast<Position*>(this), *it) << " ";
 
   return ss.str();