The position component is redundant with the built-in Bevy Transform.
I've updated the velocity integrator to use the transform directly.
Physics still works, but things still set their initial locations
through the Position component. I'll need to fix all those call sites.
The physics sim bits (that aren't Rapier2d) are now in their own
submodule. I've included the `Wrapping` marker component because it
doesn't really have anywhere else to live, and it's kinda sorta related
to the physics. It controls the motion of objects... that's physics. :p
There are name collisions between Rapier2d's `Velocity` and my own, but
that's a problem for later. I've used more specific type paths where
necessary to maintain the previous behavior.
The `physics::Position` component can go away completely. It's just an
extra copy of some of the built-in `Transform` data. I'm pretty sure it
only exists because I didn't realize I could rely on directly
manipulating the transform when I started this project.
