Merge PItemState and PItem

[ignasi35]

PItem and PItemState should be merged. This change will require JSON Evolutions which is why I think it'd better be done as a separate PR.

[TimMoore]

I don't know if we really need to worry too much about JSON evolutions/migrations at this stage. This is really a pre-1.0 thing.

[ignasi35]

I'm more interested in providing examples and getting used to the costs of refactoring serialised classes.

[TimMoore]

That's a good point, but I think we can be deliberate with our choice of examples for JSON migration, to ensure they're easy for people to understand. Maybe this is a good example, but I want to be sure we're choosing it for that reason 😄

[TimMoore]

Will this require use of the optional serializer?

[ignasi35]

Maybe not (in any case it's probably coming in akka 2.4.15 😂 ). I'm thinking we could (ab)use the ItemStatus enum for that purpose. But I haven't thought of all the implications wrt each of the fields in PItem.