Bevy for simulation

[bobbbay]

Currently, there is no simulation UI, which leads to a rather bland definition of "simulation". It would be great if the simulation program could spawn a gRPC server and a Bevy frame, at once. Furthermore, using global mutable state to keep both the server and the UI up-to-date would be useful. Finally, we could use rapier's physics plugin to simultaneously calculate physics-related calculations and render them, at once.

[bobbbay]

Finished as of 6ebe442