Replacement of es5 functions to es6 classes feat p5.polySynth (#523)

Author: endurance21

src/app.js

@@ -6,7 +6,10 @@ p5.prototype.getAudioContext = getAudioContext;
 p5.prototype.userStartAudio = userStartAudio;
 
 import './master';
-import './helpers';
+
+import { freqToMidi } from './helpers';
+p5.prototype.freqToMidi = freqToMidi;
+
 import './errorHandler';
 import './audioWorklet';
 import './panner';
@@ -66,4 +69,5 @@ p5.AudioVoice = AudioVoice;
 import MonoSynth from './monosynth';
 p5.MonoSynth = MonoSynth;
 
-import './polysynth';
+import PolySynth from './polysynth';
+p5.PolySynth = PolySynth;

src/helpers.js

@@ -27,7 +27,7 @@ p5.prototype.sampleRate = function () {
  *                             above Middle C is 440Hz
  *  @return {Number}   MIDI note value
  */
-p5.prototype.freqToMidi = function (f) {
+export const freqToMidi = function (f) {
   var mathlog2 = Math.log(f / 440) / Math.log(2);
   var m = Math.round(12 * mathlog2) + 69;
   return m;