Merge AnnotationTypeWriter[Impl] into ClassWriter[Impl]

It ought to be possible to merge AnnotationTtypeWriter[Impl] into ClassWriter[Impl].

ClassWriter[Impl] is already used to generate docs for enums, which are just a special kind of class, and so it ought to be possible to have it handle annotation types as well, in which the methods are treated as "annotation type elements", either "required" or "optional".

If nothing else, AnnotationTypeWriter[Impl] could maybe be a subtype of ClassWriter[Imp].