1) NCicTypechecker.typecheck_obj removed, since it did not add to the

1) NCicTypechecker.typecheck_obj removed, since it did not add to the
   environment (and thus all objects were type-checked at least twice)
2) NCicEnvironment.check_and_add_obj that checks and add an object
3) added NCicEnvironment.invalidate_obj that invalidate all objects after a
   given one (that comprised)
4) NCicLibrary.add_obj now calls the NCicEnvironment.check_and_add_obj;
   NCicLibrary.time_travel now calls NCicEnvironment.invalidate_obj

Note: it could happen that an object is in the environment but not in the
      library and thus it could not be invalidated by time_travel.
      In practice, this is safe anyway and it will never happen.