Signature.TypeArg to change to algebraic type

[Brian Goetz]

> Thus, I believe the impact is on the lesser end. Before we perform the 
> refactor, should we handle default, extends, super in one class + 
> 3-value enum or 3 classes (like RuntimeInvisible/Visible annotations)?

I think the enum approach is probably better suited to this situations.  
We have separate R{VI}AA classes because we decided "every attribute 
defined in the JVMS should have its own attribute subtype" (and similar 
with each kind of classfile metadata), but for something as simple as 
this an enum for variance seems more properly sized.