Re-arrange Era instances

[lehins]

Description

This PR moves all Era type class instances into the internal sublibrary of cardano-ledger-core, which ii believe is an improvement in its own right, since we can now see all eras and their expected protocol versions in one place. However, the bigger reason why this change is introduced is to improve how we define constraints for functionality that should be available up to or starting from a specific era. This is achieved by introduction of a type level name for each era, that can safely be used to infer protocol version, without exposing access to the actual era types too early in the hierarchy.

Checklist

• Commits in meaningful sequence and with useful messages.
• Tests added or updated when needed.
• CHANGELOG.md files updated for packages with externally visible changes.
  NOTE: New section is never added with the code changes. (See RELEASING.md).
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary.
  NOTE: If bounds change in a cabal file, that package itself must have a version increase. (See RELEASING.md).
• Code formatted (use scripts/fourmolize.sh).
• Cabal files formatted (use scripts/cabal-format.sh).
• CDDL files are up to date (use scripts/gen-cddl.sh)
• hie.yaml updated (use scripts/gen-hie.sh).
• Self-reviewed the diff.

[teodanciu]

I admit the relationship between the new EraHasName and Era is over my head.
But the result is great - and also find it useful that all Era definitions are now in the same place - i seem to be looking at these definitions - small as they are- quite often, its good that they're much easier to find.
Looks good to me

[aniketd]

I just have questions 😅 . Looks great!