Generate getters for ManagedObjectIds

[tpodhraski]

It would be useful to generate getters for the ManagedObjectID references. So the code which is now:

_managedContext.getObjectById(object.property):*

would become:

object.propertyRef

This has two benefits. The code becomes type safe and client code becomes less complex. To fully implement this we need ForeignObjectId (#1).

[tpodhraski]

In order to support generating getters of the correct type we have to add type information to ManagedObjectId in the models. The simplest way is just to use generics like for the ManagedArray. So:

hand: ManagedObjectId<Item>

This should not brake existing models, ManagedObjectId without specific class it's referring would equal to type any:

ManagedObjectId -> ManagedObjectId<*>