Convert killers to a vector

Add infrastructure to threat killer moves as a vector,
this will allow us to easily parametrize the number of
killer moves, instead of hardcode this value to two as is now.

This patch just add the infrastructure, no functional
change.

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

src/position.cpp

@@ -1914,8 +1914,7 @@ bool Position::is_mate() {
 
   if (is_check())
   {
-      MovePicker mp = MovePicker(*this, false, MOVE_NONE, MOVE_NONE,
-                                 MOVE_NONE, MOVE_NONE, Depth(0));
+      MovePicker mp = MovePicker(*this, false, MOVE_NONE, EmptySearchStack, Depth(0));
       return mp.get_next_move() == MOVE_NONE;
   }
   return false;