fix a translation error
kill dead code
Normalize the bat exercises to unbreak the testing infrastructure

- the logic is carried by the entities again (and not the world when
  running the exo or even the exercise itself when computing the
  answers)
- every worlds are given an entity directly by the constructors. There
  is however no need to mutate it.
- equality between bat tests is tweaked. If we detect that someone's
  doing answer.equals(current) or the contrary, we compare
  answer.expected to current.result.

  Messing with the equals method is a bit of a hack, but in our case,
  I think it makes sense.