EBB Firmware: Add a native arc command

[EmbeddedMan]

This will be for v3.0.0 firmware.

Exact syntax to be worked out, but a good starting point (thanks @oskay) is
CM, dest_x, dest_y, center_x, center_y, rate, direction
Where all of dest_x, dest_y, center_x and center_y are relative to the initial position and direction is 0 for CW and 1 for CCW.
This is basically an arc version of the existing SM (constant velocity) command.

[oskay]

Let's take the rate as StepFrequency, same as that from http://evil-mad.github.io/EggBot/ebb.html#HM

center_x and center_y should be capped at ±2^15 ( ±32,768). If center_i > 32768 set it to 32768 and if If center_i < -32768 set it to -32768.