MOMs

Fixes Issue #20, the persistent model types do not generate a cached entity anymore. Multiple ManagedObjectModels can be built from a single type and the deprecated init's for that are available again.
MOMs themselves are still cached for other methods that take PersistentModel types (like .modelContainer(for: Item.self)).

Also, the attributeValueClassName of NSAttributeDescriptions isn't filled anymore w/ the corresponding Foundation types. That produced issues w/ optional attributes (NSNull not matching the set type) and doesn't seem to be required either.