Make Null a subclass of AnyVal under -Yexplicit-nulls

[bishabosha]

@robstoll

If I have to start adding nn everywhere again (or “trust” the consumers of the function to never pass null which of course is risky) then I don’t really see the benefit.

there is a follow up proposal that basically should make those methods on Any that throw NullPointerException instead handle null gracefully when the reciever’s upper bound includes Null (for code compiled with latest compiler) - so therefore no fear of NPE when calling e.g. x.toString or x.getClass (specifically with additional wrapper code only in situations where x: T and Null <:< T)