AxisSteppers should not have to be initialized with their index

The first argument to an AxisStepper constructor is its own index within a tuple of axis steppers. Its used so that the MotionPlanner can operate on AxisStepper references, and call their index() method to map the reference back to its position in the tuple. So really, those parts of the MotionPlanner just need to be refactored to pass around both a reference and an associated index.
