Make Null a subclass of AnyVal under -Yexplicit-nulls

[sjrd]

@sjrd

The fact that (x: Any).toString()—in general, (x: T).someMethodOfAny()—is unsafe is a valid point. However, making Null not even a subtype of Any is definitely not an option for Scala, so that cannot be the solution. Therefore, this concern is orthogonal to the change proposed here. I will address it in a separate post/proposal.

Here is the follow-up: Proposal: Fixing null.methodOfAny() under explicit-nulls