message: allow PolySynth voice constructors to extend Monophonic
diff: @@ -26,6 +26,7 @@ type VoiceOptions<T> =
  T extends MonoSynth ? MonoSynthOptions :
  T extends AMSynth ? AMSynthOptions :
  T extends Synth ? SynthOptions :
  + T extends Monophonic<infer U> ? U :
  never;
  /**
  