Annotate methods and parameters for nullability

If methods and parameters were annotated for nullability (ie: either `@Nullable` or `@NotNull`) it would help Java programmers who use static analysis tools, and would be a huge help to Kotlin programmers using the Java SDK.

Nullability annotations in Java unfortunately suffer from a bit of a [too many standards](https://xkcd.com/927/) problem, but JetBrains has [a list of nullability annotations supported by the Kotlin compiler as well as other advice that should help with migrating to using nullability annotations](https://kotlinlang.org/docs/reference/java-interop.html#nullability-annotations).