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/main.cpp

@@ -52,6 +52,7 @@ int main(int argc, char *argv[]) {
 
   // Initialization
 
+  init_piece_attacks_fn();
   init_mersenne();
   init_direction_table();
   init_bitboards();