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.
Much sadness that I can't use `Joiner` here (or if I can, I couldn't figure it out). I need like a `Joiner` visitor that works with `Appendable` and gives me callbacks.