Change 4 PParam fields from EpochNo to EpochInterval

[TimSheard]

EpochNo represents a fixed point in time. Sometimes we want to talk about a positive delta in this space. We used to also use EpochNo for this purpose, but this was confusing, since the EpochNo used could be negative.

To Fix this we introduce the newtype EpochInterval, and the function
addEpochInterval :: EpochNo -> EpochInterval -> Epoch No

Fixes #3870

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]

Beautiful! Thank you!

[lehins]

I probably was not to clear when I was explaining those bounds and versions. Hopefully my comments will clarify it a bit. Let me know if it still confusing and we can talk about it on Monday

[lehins]

Few more minor changes are still needed. Looking very nice though!

[lehins]

Couple more things. Almost there.

[lehins]

Thank you for your patience and your work on this PR!