(Addresses #4876)

This pull request annotates everything in the io.reactivex.annotations package with a new @NonNull annotation defined in RxJava. This doesn’t annotate everything, but I think it’s a good start.

In particular, when migrating a large codebase of RxJava 1.x code to 2.x, catching null returns in these interfaces was very difficult. At best, it’s noticed while migrating, otherwise it’s up to unit tests or a production crash to catch these.

With these new annotations, you get much better IDE support (once you tell IntelliJ about the new annotation):

^ in this example, getPhoneNumber() is marked as @Nullable

In addition, support for this annotation can easily be added to static analysis tools like Infer, checker, or any other popular tool.

Open Questions

• Does this even make sense to add? There seem to be some concerns in the issue.
• The @NonNull annotation needs JavaDoc - I’m planning on following up with whatever is in existing NonNull annotations floating around unless anyone thinks otherwise.
• There are no tests since this is essentially just metadata. Should there be something that enforces this? I'm not sure if there is any kind of lint tool that runs on this project, but one approach would be to write a check that enforces everything in specific packages in annotated. This would also help keep future changes annotated.
• Is it worth adding this elsewhere? I find these function interfaces to be the easiest place to run into nullability problems, but I'm curious what others think. A simple, but kind of crazy brute force approach would be to require annotations on every public API.