Cardano Updates

Add the statement stub for issue #417 in a new module
Ledger.Conway.Specification.Gov.Properties.LastVoteApplied:

- votedOn / lookupGAState / recordedVote: read back the vote a voter has
  recorded on a governance action in a GovState
- lastVoteOn: the last vote a voter casts on an action in a block
- vote-applied-to-GA: a single GOV vote step records the cast vote (base case)
- last-vote-applied-to-GA: over a block (GOVS), the last vote a voter casts on
  an action is the one recorded in the resulting state

The proof is left as future work ("coming soon"). Wire the module into the
Gov.Properties aggregator, list the claim in the properties index, and note it
in the changelog.