better names for ValiumInfo helpers

* @value has become @unboxed
* isValue is now isUnboxedValiumRef
* isValiumRef is now isBoxedValiumRef

This not only fixes the conflict between Symbol.isValue and RichSymbol.isValue
(the primary motivator for the refactoring), but makes things much easier
to understand. For example, compare one of the most indicative snippets:

   if (oldTpe.isValue) unbox2box else box2unbox // before
   if (oldTpe.isUnboxedValiumRef) unbox2box else box2unbox // after