Handle enum types specially in type graphs

[smls]

In our type graphs, enum types like PromiseStatus are currently rendered like normal subtypes of their value type: http://doc.perl6.org/images/type-graph-Int.png

This can create confusion.

We should review all built-in enum types and then decide which of the following solutions is better:

• render enum types differently from classes
• exclude enum subtypes altogether from type graphs of class types

[moritz]

We should do both, IMHO. Render them differently from roles and classes, and only show them in the type graph of the enum itself.

[AlexDaniel]

I think this can be closed for now. Please open a new issue if any further improvements are desired.