Use pointer-to-members to remove a bunch of duplicated code

Remove all generate_XXX_moves() functions, use an array
of pointer to members instead.

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

src/position.h

@@ -370,6 +370,11 @@ private:
 };
 
 
+/// An array of member functions to dispatch attacks_square
+typedef Bitboard (Position::* Piece_attacks_fn)(Square s) const;
+extern Piece_attacks_fn piece_attacks_fn[7];
+extern void init_piece_attacks_fn();
+
 ////
 //// Inline functions
 ////