Factor out Position::do_capture_move()

Start to slim line count i position.cpp

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

src/position.h

@@ -297,6 +297,7 @@ private:
   void allow_ooo(Color c);
 
   // Helper functions for doing and undoing moves
+  void do_capture_move(Move m, PieceType capture, Color them, Square to);
   void do_castle_move(Move m);
   void do_promotion_move(Move m, UndoInfo &u);
   void do_ep_move(Move m);