Port lawnjelly's dynamic BVH implementation from 3.x to be used in
both 2D and 3D broadphases.
Removed alternative broadphase implementations which are not meant to be
used anymore since they are much slower.
Includes changes in Rect2, Vector2, Vector3 that help with the template
implementation of the dynamic BVH by uniformizing the interface between
2D and 3D math.
Co-authored-by: lawnjelly <lawnjelly@gmail.com>
