Includes:
- More scene parsing code. Making good progress!
- Making the rendering code actually use the Scene and Assembly
types.
- Bare beginnings of a Tracer type.
Everything is done with indices anyway, so there was no reason
for it to store an internal reference to the object data. This
gets rid of the type parameter and lifetime parameter on the BVH
struct itself, which will also make it easier to bundle it with
the data it indexes, which will be important later on.
