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

sim/state/midi.ts

@@ -0,0 +1,7 @@
+namespace pxsim.control {
+    export function __midiSend(data: RefBuffer) {
+        const b = board();
+        pxsim.AudioContextManager.sendMidiMessageAsync(data)
+            .done();
+    }
+}