`constrained-generators`: `(++.)` and `singleton_` for lists

[MaximilianAlgehed]

Description

This is a simplified approach to that of #4658 with the goal of just doing what's necessary for ++ and singleton. Some things haven't been finished yet:

• [x] clean up the implementation of propagateSpecFun
• [x] implement the fold spec cases
• [x] examples and better tests than just soundness
• [x] ensure a higher success ratio on prop_univSound $ TestableFn (appendFn @BaseFn @Int)
• [x] add syntax-level functions
• [x] comments + better error messages

Checklist

• [x] Commit sequence broadly makes sense and commits have useful messages
• [x] New tests are added if needed and existing tests are updated
• [x] 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] When applicable, versions are updated in .cabal and CHANGELOG.md files according to the versioning process.
• [x] 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)
• [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)
• [x] Self-reviewed the diff

[MaximilianAlgehed]

Nothing much to do about the formatting, but some better error messages and comments are on my mental todo list...