Cleanup some of WorkArounds and usage thereof

The doclet WorkArounds class is used to gain back-door access to internal API that is not otherwise exposed. Some of that functionality is now available on public API, and can be removed from the WorkArounds class. In some cases, the footprint on the backdoor API can be reduced.

One particularly annoying wart is the cast on DocletEnvironment to DocEnvImpl, which effectively prevents using subtypes to carry additional info. It is not clear (even now) what the best way is to replace that logic.