Fine tuning of new stepper algorithm with protected planner. Adaptive step prediction for segment buffer.

- Cleaned up the new stepper algorithm code with more commenting and
better logic flow.

- The new segment buffer now predicts the number of steps each segment
should have to execute over about 8 milliseconds each (based on the
ACCELERATION_TICKS_PER_SECOND setting). So, for when the whole segment
buffer is full, the stepper algorithm has roughly 40 milliseconds of
steps queued before it needs to refilled by the main program.

- Readjusted the max supported step rate back to 30kHz from the lower
development 20kHz. Everything still works amazing great and the test
CNC machine still runs twice as fast with the new stepper algorithm and
planner.

- Upped the standard serial baudrate to 115200 baud, as it is clear
that the bottleneck is the serial interface. Will now support this, as
well as the old 9600 baud, in new firmware builds.

defaults.h

@@ -140,9 +140,9 @@
   #define DEFAULT_Z_STEPS_PER_MM (STEPS_PER_REV*MICROSTEPS/MM_PER_REV)
   #define DEFAULT_STEP_PULSE_MICROSECONDS 10
   #define DEFAULT_ARC_TOLERANCE 0.005 // mm
-  #define DEFAULT_RAPID_FEEDRATE 2500.0 // mm/min
+  #define DEFAULT_RAPID_FEEDRATE 4000.0 // mm/min
   #define DEFAULT_FEEDRATE 1000.0 // mm/min
-  #define DEFAULT_ACCELERATION 150.0*60*60 // 150 mm/min^2
+  #define DEFAULT_ACCELERATION 400.0*60*60 // 150 mm/min^2
   #define DEFAULT_JUNCTION_DEVIATION 0.05 // mm
   #define DEFAULT_STEPPING_INVERT_MASK (1<<Y_DIRECTION_BIT)
   #define DEFAULT_REPORT_INCHES 0 // false