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.)

stepper.h

@@ -3,7 +3,7 @@
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
-  Copyright (c) 2011 Sungeun K. Jeon
+  Copyright (c) 2011-2013 Sungeun K. Jeon
 
   Grbl is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
@@ -24,6 +24,10 @@
 
 #include <avr/io.h>
 
+#ifndef SEGMENT_BUFFER_SIZE
+  #define SEGMENT_BUFFER_SIZE 7
+#endif
+
 // Initialize and setup the stepper motor subsystem
 void st_init();
 
@@ -45,10 +49,10 @@ void st_cycle_reinitialize();
 // Initiates a feed hold of the running program
 void st_feed_hold();
 
+// Reloads step segment buffer. Called continuously by runtime execution protocol.
 void st_prep_buffer();
 
-uint8_t st_get_prep_block_index();
-
-void st_fetch_partial_block_parameters(uint8_t block_index, float *millimeters_remaining, uint8_t *is_decelerating);
+// Called by planner_recalculate() when the executing block is updated by the new plan.
+void st_update_plan_block_parameters();
 
 #endif