Special proof mode class for adding a modality to a goal.

This used to be done by using `ElimModal` in backwards direction. Having
a separate type class for this gets rid of some hacks:

- Both `Hint Mode`s in forward and backwards direction for `ElimModal`.
- Weird type class precedence hacks to make sure the right instance is picked.
  These were needed because using `ElimModal` in backwards direction caused
  ambiguity.