Mesh slicing

Engines like Rapier instead only have inter-island parallelism, which means that large islands are solved serially, unlike with graph coloring. Our solver now scales better, and is even faster than Rapier for large islands when multi-threaded!
("avian2d main" is old, "avian2d graph-coloring" is new)

Implemented some overengineered physics diagnostics for Avian to monitor various timers and counters in real-time, along with a pretty UI overlay for displaying them. This is very useful for profiling and optimizing Avian itself, but will also let users see what time is being spent on.

A lot of it is still WIP, but I've made a PR for contact hooks, and will soon open one for tangent velocity and material properties that can be modified per contact manifold. The hooks and tangent velocity let you make cool stuff like these conveyor belts!
github.com/Jondolf/avia...