Add instance ToCardanoEra Exp.Era

[smelc]

Changelog

- description: |
    Add instance ToCardanoEra Exp.Era
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Required for https://github.com/IntersectMBO/cardano-cli/pull/968

Checklist

• [X] Commit sequence broadly makes sense and commits have useful messages
• [X] Self-reviewed the diff

[smelc]

Closing as this is superseded by https://github.com/IntersectMBO/cardano-api/pull/636

Thanks @carbolymer for the remark :heart: