Seems like an essential part of the toolbox -- we have Point, LineSegment
and Line and now we have also all "swept sphere" alternatives to them --
Sphere (swept "along" the point), Capsule (swept along line segment) and
Cylinder (swept along infinite line).
"Implemented" collision detection with point and sphere, which is just
simplified version of collision detection implemented for Capsule (i.e.
no need to check for cap cases).
This class now stores the tree in flat array, making it easier for user
to query the contents, but the internals are much more complicated. This
solution already reduces allocation count by count of nodes in the tree,
future work might remove the per-shape allocation altogether by using
large typeless array and placement-new etc.