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.

stepper.h

@@ -44,4 +44,9 @@ void st_prep_buffer();
 // Called by planner_recalculate() when the executing block is updated by the new plan.
 void st_update_plan_block_parameters();
 
+// Called by runtime status reporting if realtime rate reporting is enabled in config.h.
+#ifdef REPORT_REALTIME_RATE
+float st_get_realtime_rate();
+#endif
+
 #endif