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.)

config.h

@@ -80,14 +80,14 @@
 // CNC applications, the following multiplier value will work more than well enough. If you do have
 // happened to weird stepper motion issues, try modifying this value by adding or subtracting a 
 // zero and report it to the Grbl administrators. 
-#define INV_TIME_MULTIPLIER 10000000.0
+#define INV_TIME_MULTIPLIER 100000
 
 // Minimum stepper rate for the "Stepper Driver Interrupt". Sets the absolute minimum stepper rate 
 // in the stepper program and never runs slower than this value. If the INVE_TIME_MULTIPLIER value
 // changes, it will affect how this value works. So, if a zero is add/subtracted from the
 // INV_TIME_MULTIPLIER value, do the same to this value if you want to same response. 
 // NOTE: Compute by (desired_step_rate/60) * INV_TIME_MULTIPLIER/ISR_TICKS_PER_SECOND. (mm/min)
-#define MINIMUM_STEP_RATE 1000L // Integer (mult*mm/isr_tic)
+// #define MINIMUM_STEP_RATE 1000L // Integer (mult*mm/isr_tic)
 
 // Minimum stepper rate. Only used by homing at this point. May be removed in later releases.
 #define MINIMUM_STEPS_PER_MINUTE 800 // (steps/min) - Integer value only
@@ -161,6 +161,14 @@
 // up with planning new incoming motions as they are executed. 
 // #define BLOCK_BUFFER_SIZE 18  // Uncomment to override default in planner.h.
 
+// Governs the size of the intermediary step segment buffer between the step execution algorithm
+// and the planner blocks. Each segment is set of steps executed at a constant velocity over a
+// fixed time defined by ACCELERATION_TICKS_PER_SECOND. They are computed such that the planner
+// block velocity profile is traced exactly. The size of this buffer governs how much step 
+// execution lead time there is for other Grbl processes have to compute and do their thing 
+// before having to come back and refill this buffer, currently at ~50msec of step moves.
+// #define SEGMENT_BUFFER_SIZE 7 // Uncomment to override default in stepper.h.
+
 // Line buffer size from the serial input stream to be executed. Also, governs the size of 
 // each of the startup blocks, as they are each stored as a string of this size. Make sure
 // to account for the available EEPROM at the defined memory address in settings.h and for