testlib: Allow changing return type by annotation in order to change final comparison or counterexample

(for some time now)

To be used for deriving arbitrary for recursive data types.

original commits:
    CI: Try bumping GHC version
    CI: Build actions on all branches
    CI: Use multiple GHC versions
    CI: Use make, add doc
    CI: Install Python
    CI: Intall type hints
    CI: Install pyYAML
    CI: Key the cache by GHC version
    CI: Fix deploy
    CI: Polishing