Shrinking for constrained-generators

[MaximilianAlgehed]

Description

This implements shrinking for constrained-generators by embedding it in the HasSpec class. There are still some TODOs that need to be covered but this should be relatively ready to look at.

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

[MaximilianAlgehed]

It might be interesting for @Soupstraw to also have a look at this. Not required but spreading knowledge around isn't a bad thing.

[Soupstraw]

LGTM

[TimSheard]

This PR adds a new method to HasSpec, supplies the method for all the instances, and then adds some tests that the shrinking does not affect the results.

Just a few questions to answer. May be add some more comments.