Deprecate `case_` for `unit_`

Created by: lwm

The deprecation isn't so easy because of the fact that the clash between duplicate generatorClass (see test failure).

Need to make decision there. I'd like to avoid breaking peoples test code.

Remaining things to do:

• Deal with test failure and decision detailed above
• Bump major version and make Hackage release

Working on https://github.com/lwm/tasty-discover/issues/95.