Clean up constraints on Proposal and Vote instances, add their generators for tests

[carbolymer]

Changelog

- description: |
    Clean up constraints on Proposal and Vote instances, add their generators for tests
  # no-api-changes: the API has not changed
  # compatible: the API has changed but is non-breaking
  # breaking: the API has changed in a breaking way
  compatibility: compatible
  # feature: the change implements a new feature in the API
  # bugfix: the change fixes a bug in the API
  # test: the change fixes modifies tests
  # maintenance: the change involves something other than the API
  # If more than one is applicable, it may be put into a list.
  type: bugfix

Context

n/a

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
• The version bounds in .cabal files are updated
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• The changelog section in the PR is updated to describe the change
• Self-reviewed the diff

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[Jimbo4350]

Ah so this is problematic because it assumes things won't change in future eras. We can update the Vote era type with the ConwayEraOnwards era GADT, See StakeDelegationRequirementsConwayOnwards as an example.

[carbolymer]

I've changed type application to (ShelleyLedgerEra era) - that should fix the CBOR en/decoding for eras != Conway. Do you also mean that we should add a constructor parameter with type ConwayEraOnwards era?