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/winch.py

@@ -21,7 +21,6 @@ class WinchKinematics:
             self.anchors.append(a)
             s.setup_itersolve('winch_stepper_alloc', *a)
             s.set_trapq(toolhead.get_trapq())
-            toolhead.register_step_generator(s.generate_steps)
         # Setup boundary checks
         acoords = list(zip(*self.anchors))
         self.axes_min = toolhead.Coord(*[min(a) for a in acoords], e=0.)