Detect whether we need a call to "super".writeObjectImpl sanely.

The whole business is more fragile than I'd like because Java's public compiler
interface doesn't let me look at a field's initializer. If I see a final field
with an initializer, I can know to ignore it for streaming purposes. Since I
can't look at that, I am stuck just correlating constructor arg names to field
names and then ignoring any fields that don't appear to be initialized by the
constructor. It would be super awesome if I could actually see the
initialization statements in the constructor as well, but that's also not
exposed by the simplistic public AST API.
The nexus-tools jar now has com.threerings.nexus groupId.
Some updates to chat demo.