Nullability annotations continued: think of alternatives

[ljacqu]

Unfortunately, @NotNull clutters the code and especially array/varargs and makes it more difficult to follow the code. In this issue, we want to think about a long-term solution for this, such as:

• keep it as it is
• see if we can default params to be not-null (I think that's not possible -> need to make sure Kotlin picks up on it)
• migrate the project to Kotlin?
• Introduce a Kotlin facade?
• keep the notnull annotations only on API classes

[ljacqu]

This is interesting: https://github.com/google/guava/blob/master/guava/src/com/google/common/base/ElementTypesAreNonnullByDefault.java