RFC: Rethinking how we store stepping configuration

[Jeff-Ciesielski]

For the sake of simplicity, it would be nice if we made a few changes to how we store step parameters:

• Steps/mm can be stored in terms of full steps, and the 'active' steps/mm can be derived from full_steps_mm * microstepping multiplier
• Microstepping config can be stored as a whole multiplier (1, 2, 4, 16...etc) per axis rather than a mask The mask will become irrelevant with the SPI drivers and it would be nice if we had a unified way of describing this config (also, the new drivers support far more microstepping options that can't be encoded in 2 bits/axis)