Arc mm_per_segment removed, now in terms of tolerance. Stepper ramp counter variable type corrected.

- Arc mm_per_segment parameter was removed and replaced with an
arc_tolerance parameter, which scales all arc segments automatically to
radius, such that the line segment error doesn't exceed the tolerance.
Significantly improves arc performance through larger radius arc,
because the segments are much longer and the planner buffer has more to
work with.

- Moved n_arc correction from the settings to config.h. Mathematically
this doesn't need to be a setting anymore, as the default config value
will work for all known CNC applications. The error does not accumulate
as much anymore, since the small angle approximation used by the arc
generation has been updated to a third-order approximation and how the
line segment length scale with radius and tolerance now. Left in
config.h for extraneous circumstances.

- Corrected the st.ramp_count variable (acceleration tick counter) to a
8-bit vs. 32-bit variable. Should make the stepper algorithm just a
touch faster overall.

config.h

@@ -105,14 +105,6 @@
 #define CMD_CYCLE_START '~'
 #define CMD_RESET 0x18 // ctrl-x
 
-// The temporal resolution of the acceleration management subsystem. Higher number give smoother
-// acceleration but may impact performance. If you run at very high feedrates (>15kHz or so) and 
-// very high accelerations, this will reduce the error between how the planner plans the velocity
-// profiles and how the stepper program actually performs them. The correct value for this parameter
-// is machine dependent, so it's advised to set this only as high as needed. Approximate successful
-// values can widely range from 50 to 200 or more. Cannot be greater than ISR_TICKS_PER_SECOND/2.
-#define ACCELERATION_TICKS_PER_SECOND 100L 
-
 // The "Stepper Driver Interrupt" employs the Pramod Ranade inverse time algorithm to manage the
 // Bresenham line stepping algorithm. The value ISR_TICKS_PER_SECOND is the frequency(Hz) at which
 // the Ranade algorithm ticks at. Maximum step frequencies are limited by the Ranade frequency by
@@ -124,6 +116,17 @@
 // 20kHz by performing two steps per step event, rather than just one.
 #define ISR_TICKS_PER_SECOND 20000L  // Integer (Hz)
 
+// The temporal resolution of the acceleration management subsystem. Higher number give smoother
+// acceleration but may impact performance. If you run at very high feedrates (>15kHz or so) and 
+// very high accelerations, this will reduce the error between how the planner plans the velocity
+// profiles and how the stepper program actually performs them. The correct value for this parameter
+// is machine dependent, so it's advised to set this only as high as needed. Approximate successful
+// values can widely range from 50 to 200 or more. Cannot be greater than ISR_TICKS_PER_SECOND/2.
+#define ACCELERATION_TICKS_PER_SECOND 100L 
+
+// NOTE: Make sure this value is less than 256, when adjusting both dependent parameters.
+#define INTERRUPTS_PER_ACCELERATION_TICK (ISR_TICKS_PER_SECOND/ACCELERATION_TICKS_PER_SECOND)
+
 // The Ranade algorithm can use either floating point or long integers for its counters, but for 
 // integers the counter values must be scaled since these values can be very small (10^-6). This
 // multiplier value scales the floating point counter values for use in a long integer. Long integers
@@ -193,6 +196,13 @@
 // parser state depending on user preferences.
 #define N_STARTUP_LINE 2 // Integer (1-5)
 
+// Number of arc generation iterations by small angle approximation before exact arc trajectory 
+// correction. This parameter maybe decreased if there are issues with the accuracy of the arc
+// generations. In general, the default value is more than enough for the intended CNC applications
+// of grbl, and should be on the order or greater than the size of the buffer to help with the 
+// computational efficiency of generating arcs.
+#define N_ARC_CORRECTION 20 // Integer (1-255)
+
 // ---------------------------------------------------------------------------------------
 // FOR ADVANCED USERS ONLY: