Ensure types.maybe(types.reference()) types are resolved late after construction

This fixes a bug in the fast instantiator code where valid references would throw a `can't resolve reference` error due to the instantiation order within MQT. We were accidentally eagerly resolving some references when constructing a tree, instead of late resolving them at the end like we normally do. This only happened for wrapped reference types like a `maybe(reference())` or a `late(reference())` because of an instanceof check instead of a deeper `isReferenceType` check.

Fixes #78, which was an overly broad bug report as it turns out -- class models and normal models correctly late resolve references, regardless of property definition order before this change, it's just when you wrap the reference in a maybe that things got wonky.

Author: airhorns