Add Pulse waveform and pulse width modulation (PWM) via input, applicable to all oscillator waveforms

[bjeanes]

• Display oscillator frequency in terms of BPM. Each waveform has one logical "peak" per cycle. That cycle is in terms of time. Those peaks can be considered beats, so this just lets you set a frequency in terms of beats for doing percussive/clock effects vs tone manipulation.
• Add Pulse waveform, which is high for 20% of its cycle, and zero for the rest, unless a different PWM threshold is provided.
• Allow configurable output width via PWM terminal input. Positive pulse widths mask all signal past the threshold. Negative pulse widths do the same, but they invert the signal.

This might unhelpfully conflate some concepts that we'd rather keep separate. I'm loosely looking at real world oscillators, such as https://www.synthesizers.com/q106a.html and noting the presence of pulse width control on them.

I think whether or not this merges, there should definitely be separate Clock and PWM modules with their own frequency controls. But, this could continue in some form sharing some of the mechanics.

So... totally open to this being not a useful direction!