[lworld] The hierarchy between V$ref and V$val must be sealed.

The translation scheme in https://cr.openjdk.java.net/~briangoetz/valhalla/sov/04-translation.html calls for V$ref to be a sealed super type that only permits V$val as a subtype.

The use of sealing here means that any value of class C$ref will either be a reference to a value of C$val, or will be the null reference (effectively, C$ref is the adjunction of C$val with the distinguished value null.)