Yet another major stepper algorithm and planner overhaul.

- Overhauled the stepper algorithm and planner again. This time
concentrating on the decoupling of the stepper ISR completely. It is
now dumb, relying on the segment generator to provide the number of
steps to execute and how fast it needs to go. This freed up lots of
memory as well because it made a lot tracked variables obsolete.

- The segment generator now computes the velocity profile of the
executing planner block on the fly in floating point math, instead of
allowing the stepper algorithm to govern accelerations in the previous
code. What this accomplishes is the ability and framework to (somewhat)
easily install a different physics model for generating a velocity
profile, i.e. s-curves.

- Made some more planner enhancements and increased efficiency a bit.

- The changes also did not increase the compiled size of Grbl, but
decreased it slightly as well.

- Cleaned up a lot of the commenting.

- Still much to do, but this push works and still is missing feedholds
(coming next.)

planner.h

@@ -35,8 +35,8 @@ typedef struct {
   // Fields used by the bresenham algorithm for tracing the line
   // NOTE: Do not change any of these values once set. The stepper algorithm uses them to execute the block correctly.
   uint8_t direction_bits;            // The direction bit set for this block (refers to *_DIRECTION_BIT in config.h)
-  int32_t steps[N_AXIS];             // Step count along each axis
-  int32_t step_event_count;          // The maximum step axis count and number of steps required to complete this block. 
+  int32_t steps[N_AXIS];            // Step count along each axis
+  int32_t step_event_count;         // The maximum step axis count and number of steps required to complete this block. 
 
   // Fields used by the motion planner to manage acceleration
   float entry_speed_sqr;             // The current planned entry speed at block junction in (mm/min)^2
@@ -44,10 +44,11 @@ typedef struct {
                                      //   neighboring nominal speeds with overrides in (mm/min)^2
   float max_junction_speed_sqr;      // Junction entry speed limit based on direction vectors in (mm/min)^2
   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
+  float acceleration;                // Axis-limit adjusted line acceleration in (mm/min^2)
+  float millimeters;                 // The remaining distance for this block to be executed in (mm)
   
 } plan_block_t;
+
       
 // Initialize the motion plan subsystem
 void plan_init();
@@ -64,13 +65,11 @@ void plan_discard_current_block();
 // Gets the current block. Returns NULL if buffer empty
 plan_block_t *plan_get_current_block();
 
+// Called periodically by step segment buffer. Mostly used internally by planner.
 uint8_t plan_next_block_index(uint8_t block_index);
 
-plan_block_t *plan_get_block_by_index(uint8_t block_index);
-
-float plan_calculate_velocity_profile(uint8_t block_index);
-
-// void plan_update_partial_block(uint8_t block_index, float millimeters_remaining, uint8_t is_decelerating);
+// Called by step segment buffer when computing executing block velocity profile.
+float plan_get_exec_block_exit_speed();
 
 // Reset the planner position vector (in steps)
 void plan_sync_position();