Set `Hint Mode` for `pretty`.

Title says it all.

This MR avoids issues like actris@3bab78ba, where pretty was used in an ambiguous way.

Due to the recent pretty changes, the instance that was picked changed from nat to Z.

merged

mentioned in commit fb289f0b

@iris-users If you're using pretty, as a consequence of this change, you might need some additional type annotations.