Epoch interval instances

[lehins]

Description

• Remove Num instance (See a related ticket: Move the Num instance for EpochNo and EpochSize cardano-base#455)
• Add ToCBOR/FromCBOR

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

[TimSheard]

This looks good. Originally there was no Num instance for EpochInterval, but one for EpochNo. This required a lot of code like "3" becomes "EpochInterval 3"
So while a bit more code is required, it does add some safety guarantees.