Collection objects erroneously serialize as arrays

[warriordog]

These types are all converting into JSON arrays instead of objects, because they implement IEnumerable<T>:

• ASCollection
• ASOrderedCollection
• ASCollectionPage
• ASOrderedCollectionPage

This is due to an unresolved limitation in System.Text.Json. For now, we can just remove the interface and add a call to .Items at all call sites. Some uses may require new null handling, since it's possible for a non-null object to have a null Items collection.