MIDI support in simulator (#1332)

* gluing to support MIDI devices in the simulator

* sim support for midi

* simplify interface

* ensure helper function does not show up in TS

* bump pxt

libs/core/control.cpp

@@ -302,4 +302,13 @@ namespace control {
     int deviceSerialNumber() {
         return microbit_serial_number();
     }
+
+    /**
+    * Informs simulator/runtime of a MIDI message
+    * Internal function to support the simulator.
+    */
+    //% part=midioutput block
+    void __midiSend(Buffer buffer) {
+        // this is a stub to support the simulator
+    }
 }