Draft: Use era type class constraints in generators

[locallycompact]

Draft. I tried to change some generator to use type classes rather than witnesses, which works - but I struggled to update the tests that use generators in any era. I managed to make it compile, but what I had to do is very unsatisfying, so I'm wondering if anyone can see how to do that part better - or explain why it isn't possible.

Thanks.

Changelog

- description: |
    Change hedgehog generators to use type classes.
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Additional context for the PR goes here. If the PR fixes a particular issue please provide a link to the issue.

How to trust this PR

Highlight important bits of the PR that will make the review faster. If there are commands the reviewer can run to observe the new behavior, describe them.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff