Generalized heightmap collision

[kvark]

Branching off from #773

I'm looking at the code for Heightmap, trying to see how this would be with a different kind of heightmap (that is put on top of a cylinder instead of a plane).
I see that PersistentQueryDispatcher::contact_manifolds simply goes through contact_manifolds_heightfield_shape and contact_manifolds_heightfield_composite_shape for heightfields. These methods seem general enough.

Would it be reasonable to expose them behind a trait? Just like as_support_map exposes a generic interface trait SupportMap, can we have as_heightmap exposing some trait Heightmap?