Add mappings to Agda types

[Soupstraw]

Description

Adds the SpecCastable type class that maps the implementation types to spec types.

Resolves task 2 of #3811

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

[lehins]

Bunch of little suggestions, otherwise looks great!

One thing I am not totally sold on is the name SpecCastable. This is more of a translation that can fail, rather than casting of one type to another. SpecTrans maybe? In any case, it's up to you, this is also only a suggestion