Performance optimization grab bag

[jonikorpi]

• Various booleans like hasDecay and hasSustain could be computed while creating the instrument, instead of during playback
• Scheduler has to iterate through quite a lot of each track on every call, due to having to find where extended notes end. (Because of alternate it’s hard to determine beforehand when a note will end.) There’s definitely a better way to do this, but I haven’t found it yet.
• Vibrato oscillator is created even if no oscillator in the instrument has vibrato effects
• If all oscillators have the same vibrato settings, there is no need to create a separate GainNode for each of them