Convert `small-steps-step` into a sublibrary

[Lucsanszky]

Description

Resolves #4068

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)
• [x] Code is formatted with fourmolu (use scripts/fourmolize.sh)
• [x] Cabal files are formatted (use scripts/cabal-format.sh)
• [x] hie.yaml has been updated (use scripts/gen-hie.sh)
• [ ] Self-reviewed the diff