Use a Data.OMap.Strict to replace ProposalsSnapshot

[aniketd]

Description

Closes #3789

• Merge Prevent duplicate certs and proposals #3779 first.

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]

There is an unfortunate problem with the decision of using a SSeq of v instead of SSeq of k. I commented on the actual location where the problem is. We can discuss tomorrow some possible solutions

[lehins]

Sorry, I didn't look closely enough last time. A whole bunch more comments.

[lehins]

Dammit, I reviewed the PR last night, but forgot to hit Submit Review

Looks much better. Couple of minor points still exist, but they should be easy to fix

[lehins]

Perfect! 👍