Indicate that tag 258 is optional for OSet. Fix rational CDDL

[lehins]

Description

• Removed mock/crypto.cddl, added optional tag to sets #3860 added optional 258 tags to sets, but not the oset in CDDL. This PR fixes that ommission
• Fix definition of rational in CDDL. It is no longer non-negative
• Remove positive_rational in favor of new nonnegative_rational, since that is the actual type that is used in PParams
• Switch ppCommitteeMaxTermLength to EpochNo, rather than Natural

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]

Looks good, but I question why we changed from Natural to EpochNo

[teodanciu]

Looks good to me