add unsafeMkProposals to be used for testing

[MaximilianAlgehed]

Description

When profiling prop_GOV that I'm working on over on the branch PR-fix-prop_GOV I found that after fixing a bunch of performance bottle necks in constrained-generators (now running many X faster!) checkInvariantAfterAddition was taking 24% of the time. This seemed excessive as the whole point of the constraint-based generator is to generate correct stuff by default.

This PR adds unsafeMkProposals to make it possible to side-step checkInvariantAfterAddition when generating test data.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[teodanciu]

Looks good to me