[WIP] Improve and complete Mirror API

[smarr]

[This is a work-in-progress PR, mostly to have an issue to work on, and to keep notes on some existing patches.]

The Mirror API needs to be made complete and consistent. Currently it is not even possible to navigate the basic relations in object model, for instance superclass relationships as well as class/metaclass relations.

The attached patch and example program shows how it could be used. But, the current mirror design is not yet good. Method names are ambiguous, sometimes it is unclear on what a mirror works, and generally how they are used to travers an object model.