#97 Typecheck level 0 object invariants

[Curtis Clifton]

From the JML Ref Manual: Object invariants, that is an invariant (see
section 8.2 Invariants) that is either written in an interface using the
modifier instance (see section 6.2.7 Instance) or one that is written in a
class and that does not use the modifier static (see section 8.2.1 Static
vs. instance invariants).