Tools for constrained generation of types that need witnessing

[TimSheard]

This is a work in progress. Comment requested to be sure we build a usable set of tools.

When we write generators the TxBody, we need to make sure that some of the types generated can be witnessed.
Most types that need witnessing are hashes, just generating a random hash will not do, since that hash has to be the
hash of an actual object that we can compute, given the hash. Since that usually impossible, we will have do it backwards,
i.e. generate the object, then return the hash, and remember the association for when it comes time to produce the witness.

This is the role of the data WitUniv.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• 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)
• 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)
• 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

[lehins]

Just a few minor suggestions and questions