Max velocity axes independence installed. Fixed intermittent slow trailing steps. Timer0 disable fix.

- Maximum velocity for each axis is now configurable in settings. All
rapids/seek move at these maximums. All feed rates(including rapids)
may be limited and scaled down so that no axis does not exceed their
limits.

- Moved around auto-cycle start. May change later, but mainly to ensure
the planner buffer is completely full before cycle starting a streaming
program. Otherwise it should auto-start when there is a break in the
serial stream.

- Reverted old block->max_entry_speed_sqr calculations. Feedrate
overrides not close to ready at all.

- Fixed intermittent slow trailing steps for some triangle velocity
profile moves. The acceleration tick counter updating was corrected to
be exact for that particular transition. Should be ok for normal
trapezoidal profiles.

- Fixed the Timer0 disable after a step pulse falling edge. Thanks
@blinkenlight!

main.c

@@ -103,6 +103,11 @@ int main(void)
     protocol_execute_runtime();
     protocol_process(); // ... process the serial protocol
     
+    // When the serial protocol returns, there are no more characters in the serial read buffer to
+    // be processed and executed. This indicates that individual commands are being issued or 
+    // streaming is finished. In either case, auto-cycle start, if enabled, any queued moves.
+    if (sys.auto_start) { st_cycle_start(); }
+    
   }
   return 0;   /* never reached */
 }