RefObject and ValObject

[Brian Goetz]

> V <: V? by value set inclusion; V? is the type obtained by adjoining `null` to the value set of V.  

Looking for.a better name for this.  :”Nullable value types” is a terrible name, so I don’t want to say that.  (Too confusing with null-default value types.). They could properly be define as “null-adjoined value types”, but that’s not helpful if you don’t know what an adjunction is.  Similarly for “nullable projection”.  

But the basic idea is that V? is the denotation of the union type of V | Null.