Reduced coordinates / featherstone for joints

Use something like the [Featherstone algorithm](https://en.wikipedia.org/wiki/Featherstone%27s_algorithm) to calculate joint state in reduced coordinates - one coordinate for each rotational/translational degree of freedom for each joint.

This would be a major rewrite and has a number of subtasks - the first of which is to determine whether the underlying algorithms (mass matrix calculation, forward kinematics, etc) are all XLA/TPU-friendly.