Skip parsing class members when not needed

[nikomatsakis]

In #102, @fpoli suggested that we could be more robust if we did not parse class members when we don't need to:

When the user declares an opaque class (i.e. with an empty body), the reflection checks of Duchess should just parse the class signature in the output of javap (i.e. everything that is before {), not also its body. This way, Duchess would be more robust.