Enumerator style API for DisplayConfig

[JeroMiya]

The C++ wrappers for the display config API let you enumerate the viewers, eyes, surfaces, and display input objects in a more natural way than the current managed bindings, which are flatter and closer to the C API.

Look into creating Viewer, Eye, Surface, and DisplayInput objects, and all of the associated methods for each, as well as the IEnumerable returning methods. This would make the API much easier and clean to use.

Once this is done, tag the "flat" API methods as [Obsolete] for now. They can be removed sometime before the first 1.0 release, once the Unity code is updated to use them.

[rpavlik]

:) this would be swell. the flat model is both useful (don't have to allocate memory or sling pointers around too much) but also a pain (those are some nearly-unmanageable function names).

[JeroMiya]

If I'm careful about figuring out which bits are static for the lifetime of the DisplayConfig and which bits can change, I may be able to cache some of the objects created in order to eliminate new allocations in most cases. Not sure what pieces can change - I'd have to review the docs again.

[rpavlik]

Should hopefully be clear from the docs, but the topology of objects itself is constant.