constrained-generators: Fix bug in toPreds for maps + add additional tests

[MaximilianAlgehed]

Description

There was a silly bug in toPreds whereby the required elements of the domain would become the universe of the domain when you do toPreds. This resulted in maps being constrained to be empty under unfortunate circumstances.

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

[TimSheard]

A one line change, a new rewrite rule, and some tests.
Looks good to me.