feat(UnionConverter): Support upconvert from string to nullary case

Support upconversion from a representation of an event starts with `TypeSafeEnumConverter` for a given union (and hence has a `string` value), being able to implicitly upconvert from that if the internal representation adds a non-nullary case (and hence switches to using UnionConverter).

Of course, in general one should have a clear picture of whether something is an enum or a full state with data per case. The point here is to be able to represent data that's specific to a case without that having to stuff it into adjacent optional fields with conditional logic defining when the field should be present etc.

#97 is slightly related, but is going the other direction, and is recovering from a missing converter rather than a natural evolution

Start by locating TypeSafeEnumConverter and UnionConverter and read how their string and union representations are handled. Add coverage for upconverting an existing string representation when a union gains a non-nullary case, and verify that the converter accepts the evolved representation without breaking existing cases.

Vom Indexierungsmodell aus dem Issue-Text verfasst.