Cardano Updates

This replaces the `supportedVersions` method in `EnumGolden` and gives
additional flexibility. For examples, this will be needed to add new
constructors to snapshot types. Since these did not exist for older
snapshot version, we only want to enumerate them for specific version.

The roundtrip tests only decode values that have been encoded at the
current version. Similarly, the golden file tests only check that our
serialisation doesn't change unexpectedly. This is useful, but so far
nothing tested that we can actually *decode* at older versions.

We should at least have tests for deserialisation of old versions of
specific types that changed between versions. This commit adds the first
such test.