Skip re-writing the same values to components, to satisfy bevy's change detection.

[RJ]

At the moment, a dynamic body (that isn't sleeping) with zero Linear and Angular velocity will still get its Position, Rotation, LinearVelocity, and AngularVelocity updated every tick (re-set to the same values).

This shows up if you use a Query<Entity, Changed<Position>> query filter, for example.

These commits guard against updating the components unless the value will actually change.

I noticed this while debugging some netcode that serializes and sends changed component values.

[RJ]

adds some verbosity, happy to make any style / naming changes or whatever you suggest.

[Jondolf]

I left some comments on style + an error in the velocity solver, otherwise this looks good!