More '%' modulo opertor removals and some housecleaning.

- Serial functions contained quite a few modulo operations that would
be executed with high frequency when streaming. AVR processors are very
slow when operating these. In one test on the Arduino forums, it showed
about a 15x slow down compared to a simple if-then statement. -
Clarified some variable names and types and comments.

gcode.c

@@ -328,7 +328,7 @@ uint8_t gc_execute_line(char *line) {
       }
       
       // Set clockwise/counter-clockwise sign for mc_arc computations
-      int8_t isclockwise = false;
+      uint8_t isclockwise = false;
       if (gc.motion_mode == MOTION_MODE_CW_ARC) { isclockwise = true; }
 
       // Trace the arc