MovePicker: introduce per square MVV/LVA ordering

Just added the infrastructure, no functional change.

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

src/movepick.h

@@ -77,11 +77,14 @@ private:
   void score_qcaptures();
   Move pick_move_from_list();
   int find_best_index();
+  int MovePicker::find_best_index(Bitboard* squares, int values[]);
 
   const Position& pos;
   Move ttMove, mateKiller, killer1, killer2;
   Bitboard pinned, dc;
   MoveStack moves[256], badCaptures[64];
+  Bitboard capSquares;
+  int capSqValues[64];
   bool pvNode;
   Depth depth;
   int phaseIndex;