G54 work coordinate support (w/ G10,G92.1). Re-factored g-code parser with error checking. Minor compiler compatibility changes.

- G54 work coordinate system support. Up to 6 work coordinate systems
(G54-G59) available as a compile-time option.

- G10 command added to set work coordinate offsets from machine
position.

- G92/G92.1 position offsets and cancellation support. Properly follows
NIST standard rules with other systems.

- G53 absolute override now works correctly with new coordinate systems.

- Revamped g-code parser with robust error checking. Providing user
feedback with bad commands. Follows NIST standards.

- Planner module slightly changed to only expected position movements
in terms of machine coordinates only. This was to simplify coordinate
system handling, which is done solely by the g-code parser.

- Upon grbl system abort, machine position and work positions are
retained, while G92 offsets are reset per NIST standards.

- Compiler compatibility update for _delay_us().

- Updated README.

motion_control.h

@@ -25,10 +25,6 @@
 #include <avr/io.h>
 #include "planner.h"
 
-// NOTE: Although the following function structurally belongs in this module, there is nothing to do but
-// to forward the request to the planner.
-#define mc_set_coordinate_offset(x, y, z) plan_set_coordinate_offset(x, y, z) 
-
 // Execute linear motion in absolute millimeter coordinates. Feed rate given in millimeters/second
 // unless invert_feed_rate is true. Then the feed_rate means that the motion should be completed in
 // (1 minute)/feed_rate time.