Keep Coin as the user-facing name of functions

[smelc]

Changelog

- description: |
    Add `Lovelace` as a type synonym to `Coin`
# 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

• Revert of https://github.com/IntersectMBO/cardano-api/pull/611
• A type synonym is added to show that the old Lovelace is now just the ledger's Coin

How to trust this PR

1. Only adds a type synonym
2. Removes a private function
3. New type synonym is correctly exported

Checklist

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

[smelc]

Thanks @carbolymer for the swift review!

cc @Jimbo4350