title: AKMIDICallbackInstrument implementation issue
body: <p>Updating to the latest version of AudioKit left me changing several AKCallbackInstrument instances over to the new AKMIDICallbackInstrument class which now incorporates the former as legacy behavior. When doing so however, I ran into this weird error. Maybe a Swift nuance I am missing?</p>
  
  <pre><code>let callback = AKMIDICallbackInstrument() { status, note, velocity in
      if status == .noteOn {  //errors out
         // do something
      }
  }
  </code></pre>
  
  <p>Comparing <strong>status</strong> to <strong>.noteOn</strong> errors out with:
  "Expression type 'Bool' is ambiguous without more context.". Makes sense, because AKMIDICallbackInstrument is not returning an AKMIDIStatus in status anymore, but a straight MIDIByte (UInt8). Using direct MIDI command numerics works. </p>
  
  <pre><code>let callback = AKMIDICallbackInstrument() { status, note, velocity in
      if status == 0x90 {
         // do something
      }
  }
  </code></pre>
  
  <p>So we have a problem and a potential solution. I'm just not sure that this is the way to go and AKMIDICallbackInstrument didn't hit the docs yet.</p>