Support multi-member annotations.

The controversial part of this PR is likely the testing strategy of `AnnotationWriter`. Not sure if you had ideas on how to test writers without a whole infrastructure of `JavaWriter` (and descendants) but this was my quick solution to get it working.
Heh.  It's sort of a joiner of joined things, or a multi-stage join.  It might be interesting to create a Multijoiner to handle this case, but in truth, I think this reads reasonably clearly.