Remove checker qual and migrate to jetbrains annotations

[kezz]

Closes #386.

This also removes the compile only dependencies on checker qual, not sure if these should be left until a breaking release or not.

Some points of note:

• Some @Nullable annotations were added where they were missing in equals overrides.
• There is no replacement for @MonotonicNonNull or @PolyNull. Both have been replaced with @Nullable annotations with appropriate @Contract annotations for poly null methods.

[kashike]

org.jetbrains.annotations.UnknownNullability?

[kezz]

org.jetbrains.annotations.UnknownNullability?

I pondered that but it's not really true, we do know the nullability but there's no way to accurately express it. That being said, it is definitely the case that @UnknownNullability will prevent IDE warnings (IDEA doesn't bothing nagging you) but at the cost of people potentially missing errors in their own code. I think it might make more sense to leave it @Nullable and let people ignore this warning if they know that the supplier never returns null.

[kashike]

I think it might make more sense to leave it @Nullable and let people ignore this warning if they know that the supplier never returns null.

I would rather we drop the annotation - I know that having this @Nullable would annoy people, myself included.

[kezz]

I would rather we drop the annotation - I know that having this @Nullable would annoy people, myself included.

Okie dokie! I've moved it over to @UnknownNullability as that is the default for non-annotated types and the docs for that method state pretty much the exact reason we'd use it in these supplier cases.

[kashike]

Rebase required.