- dummy index renamer didn't account for internal dummy indices of objects

- dummy index renamer didn't account for internal dummy indices of objects
  in products; a~mu.mu-a~nu.nu gets simplified to 0 now
- made a little more use of STL facilities for exvectors, especially in the
  indexed stuff; append_exvector_to_exvector() and index_set_difference() are
  gone and utils.h defines the functors ex_is_less and ex_is_equal