Reference-default style

[Brian Goetz]

> (To remind everyone:  We are using two half-buckets rather than one bucket
> mainly so that Optional can be migrated.  If it were just supporting V? then
> we’d use an empty marker type, I think, probably just an interface.)

The two half buckets also exist because it is how we get primitives and 
inlines to be the same thing, and not end up with THREE kinds of types.