Use new APIs in the core

libs/microbit/control.cpp

@@ -127,10 +127,7 @@ namespace control {
     //% help=control/in-background
     //% blockId="control_in_background" block="run in background" blockGap=8
     void inBackground(Action a) {
-      if (a != 0) {
-        incr(a);
-        create_fiber((void(*)(void*))action::run, (void*)a, fiberDone);
-      }
+      runInBackground(a);
     }
 
     /**