Stop using UMap in favor of Accounts type family

[lehins]

Description

Resolves #4694

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]

Did a first pass, will have another look later! Looks great, I finally feel I can be friends with DState 😆

[aniketd]

Just some minor corrections

[teodanciu]

Added a few more minor suggestions

[teodanciu]

That a huge piece of work!! I think it's a great improvement in terms of understanding not only what we store/map but also how to work with it, thanks to the clear interface.

I admit I got lost with the Transition modules, I don't understand at all what they're about, but that's on me, I shall study them.

Looks good to me, great work!