Reinstated feed holds into new stepper algorithm and planner. Rough draft, but working.

- Reinstated the feed hold feature with the new stepper algorithm and
new optimized planner. It works, but will be re-factored a bit soon to
clean up the code.

- At this point, feedrate overrides may need to be installed in the
v1.0 version of grbl, while this version will likely be pushed to the
edge branch soon and pushed to master after the bugs have been squashed.

- Measured the overall performance of the new planner and stepper
algorithm on an oscilloscope. The new planner is about 4x faster than
before, where it is completing a plan in around 1ms. The stepper
algorithm itself is minutely faster, as it is a little lighter. The
trade-off in the increased planner performance comes from the new step
segment buffer. However, even in the worse case scenario, the step
segment buffer generates a new segment with a typical 0.2 ms, and the
worse case is 1ms upon a new block or replanning the active block.
Added altogether, it’s argubly still twice as efficient as the old one.

planner.h

@@ -46,7 +46,7 @@ typedef struct {
   float nominal_speed_sqr;           // Axis-limit adjusted nominal speed for this block in (mm/min)^2
   float acceleration;                // Axis-limit adjusted line acceleration in (mm/min^2)
   float millimeters;                 // The remaining distance for this block to be executed in (mm)
-  
+  // uint8_t max_override;           // Maximum override value based on axis speed limits
 } plan_block_t;
 
       
@@ -75,7 +75,7 @@ float plan_get_exec_block_exit_speed();
 void plan_sync_position();
 
 // Reinitialize plan with a partially completed block
-void plan_cycle_reinitialize(int32_t step_events_remaining);
+void plan_cycle_reinitialize();
 
 // Returns the status of the block ring buffer. True, if buffer is full.
 uint8_t plan_check_full_buffer();