Realtime rate reporting. Updated decimal places.

- Added a new optional compile-time feature for ‘realtime’ (within
50ms) feed rate reporting. When querying for a status report, a new
data value will state the current operating rate. It’s only beta at the
moment and has some kinks to work out.

- Updated the code for printing floating point values to N decimal
places. Generalized the main floating point print code to accept a new
decimal places value and created a set of handler functions to print
certain floating point value types used in Grbl, like position, rates,
coordinate offsets, etc. All of these have different decimal
requirements and change when printed in mm or inches mode.

- Number of decimal places for the different value types can be
re-defined in config.h, but there shouldn’t be a need for this, as
these are physically limited.

- Removed the decimal places settings, as this was now obsoleted by the
new decimal places code.

- The new decimal places code also saves almost 300kB in flash space,
as it’s more efficient.

settings.h

@@ -70,7 +70,6 @@ typedef struct {
   uint8_t stepper_idle_lock_time; // If max value 255, steppers do not disable.
   float junction_deviation;
   float arc_tolerance;
-  uint8_t decimal_places;
   uint8_t flags;  // Contains default boolean settings
   uint8_t homing_dir_mask;
   float homing_feed_rate;