motion_queuing: Add new module to help with motion queues and flushing

Create a new module to assist with host management of motion queues.
Register all MCU_stepper objects with this module and use the module
for step generation.

All steppers will now automatically generate steps whenever
toolhead._advance_flush_time() is invoked.  It is no longer necessary
for callers to individually call stepper.generate_steps().

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/kinematics/cartesian.py

@@ -36,7 +36,6 @@ class CartKinematics:
                         'safe_distance', None, minval=0.))
         for s in self.get_steppers():
             s.set_trapq(toolhead.get_trapq())
-            toolhead.register_step_generator(s.generate_steps)
         # Setup boundary checks
         max_velocity, max_accel = toolhead.get_max_velocity()
         self.max_z_velocity = config.getfloat('max_z_velocity', max_velocity,