Support id as string about kakapo

[MP0w]

@devlucky/kakapo-core

[zzarcon]

Yes, that's why I suggested devlucky/Kakapo.js#24.

And basically allow the user to specify the type of id (we can go with autoincrement as default behaviour)

[MP0w]

I would say that Int will be the default and Storable will use the current implementation (Int). Then We can create a simple protocol CustomStorable or StringStorable or whatever
that has a default implementation that convert the Int to String and add a computed property:

var id: String {
    get {
       return String(id) // the Int id
    }
}

[MP0w]

Unfortunately we might need special handling in the Serializer to avoid using the Int in the JSON so maybe it should be the other way around

[MP0w]

@devlucky/swift-core let's discuss before deciding if it should be in the Milestone... looks like we are not sure if we want this (cc/ @zzarcon )