TODO

- Implement warm starting. This likely means contact constraints need to persist between frames.
    - Option 1: Implement contact persistance.
        - Option 1: If both bodies haven't moved much, than just assume their current contacts correspond to last frame's contacts.
        - Option 2: Use some kind of per contact proximity matching. 
    - Option 2: Instead store the warming impulse on the body. 
        - To avoid bodies jumping when a load is removed, we would need to project the warming impulse to the body's current constraint's span.

- Implement more realistic and stable friction. Support static friction.

- Improve velocity solver stability. Use box stacking as a test of stability.
    - Option 1: Explore block solvers further to see if they can yield better stability than using three contacts per touching face.
    - Option 2: Apply some damping to impulses each iteration. 
        - This seems to work well for the positional solver. It seems to allow convergence to a decent compromise solution.
        - This approach requires restitution to be applied in a final step rather than each iteration.

- Remove vector mutation in a new branch and compare performance.