Make Null a subclass of AnyVal under -Yexplicit-nulls

Indeed - i was inquiring for a real evidence based reason that a type that excludes null should be needed - that is not based on “null is ugly” - when the proposal is to make null behave more like () in a generic context (i.e. a useless value that should no longer throw NPE unexpectedly.)

however - this is probably more suited to the other thread