BVH trait definition is hard to implement when bounding volumes are generated on demand

My use case is that I have a voxel octree which needs to be very memory efficient. So I can't store an AABB for every single voxel. That's 6 floats, i.e. 24 bytes per voxel.

Rather, voxel presence is encoded in bitsets, and the efficiency is such that a single voxel can be stored in far less than 1 bit per voxel.

So the issue is that when I tried to `impl BVH` for this octree, I could not implement `fn content(&self, node: Self::Node) -> (&BV, Option<&T>)` because it returns a reference to a bounding volume. I could easily generate an `AABB` for a given voxel, but it would not live long enough to return a reference. The only hack I can think of is to keep an `AABB` temporary variable on `self` that gets unsafely mutated to overcome `&self`, but that may be unsound depending on the code that's using this structure.