Fix bugs in introspecting a class with a 'stop class':
Fix bugs in introspecting a class with a 'stop class':

- Incorrectly using Class.isInstance() instead of Class.isAssignableFrom()
  when checking the stop class
- The stop class is an exclusive bound, not an inclusive one
- Interfaces cannot have a stop class (at least that's how JDK behaves)

kaffe/kaffe,kaffe/kaffe,kaffe/kaffe,kaffe/kaffe,kaffe/kaffe