Add BlockBiquad shelf & peaking, AudioFilter support

[jepler]

@relic-se @gamblor21 @todbot for interest.

A potential limitation is:

Using the same LFO as an input to multiple other LFOs or Notes is OK, but the result if an LFO is tied to multiple Synthtesizer[sic, someone should fix that doc typo] objects is undefined.

(besides which, LFOs can now be used with AudioFilters, BlockBiquads, and other types of objects; LFOs, Math blocks and BlockBiquads should in the medium term be moved out of synthio into their own modules and the documentation updated to reflect what they can be used with)

AudioFilter is more likely to be used with AudioMixer. But you can't have one LFO drive the filters of two different AudioFilter objects.

I honestly don't know what the fix for this is. The lfo_tick routine needs to run once every SYNTHIO_MAX_DUR samples, but other parts of the audio pipeline produce or consume in different chunks. Using AudioMixer to gather up multiple chunks of synthio data is a common trick, but you still want the LFOs to tick every 256 samples even if AudioMixer is gathering 1024 samples at a time (ssay) In fact, this is making me doubt that a BlockBiquad with varying inputs is even working right in Filter with a single Filter and no mixer since it is using buffer_size of 512 bytes which might or might not be 256 samples.

This needs more thought, so I'm entering this as a draft.

[jepler]

[two builds overflowed, looks like]

[relic-se]

@jepler The updates look great to me so far, and I'm happy with the updates to audiofilters.Filter. Namely, the rewrite of common_hal_audiofilters_filter_set_filter and removal of reset_filter_states.

[jepler]

@relic-se can you comment on this specifically?

this is making me doubt that a BlockBiquad with varying inputs is even working right in Filter with a single Filter and no mixer since it is using buffer_size of 512 bytes which might or might not be 256 samples.

[relic-se]

@jepler Since the synthio.BlockInput system relies on an actively running instance of synthio.Synthesizer to operate, it isn't currently possible to use audiofilters.Filter without also using audiomixer.Mixer with two voices, one reserved for synthio.Synthesizer (even if it isn't doing anything other thank BlockInput calculations).

Here's an example of this in action: https://gist.github.com/relic-se/77e48df1920b20d3e50762cbc06d1bb9#file-audioeffects_microphone-py-L111-L112.

If synthio.BlockInput is separated from synthio.Synthesizer in the future, I think that's something that we can dive deeper into. I suspect that a static counter of some sort on whatever updates synthio.BlockInput would be used to mitigate this problem.

The example referenced relies on this PR, #9776, for the audiofilters.Distortion effect. I haven't marked that PR as ready for review yet because it still needs some additional optimization on per-sample calculations.

[jepler]

Just like using an LFO, math, or other object with a block input with two synthesizer objects is undefined, so is using an LFO with a synthesizer and an audiofilter...:

//|     Using the same LFO as an input to multiple other LFOs or Notes is OK, but            
//|     the result if an LFO is tied to multiple Synthtesizer objects is undefined.          

so sadly I have to say that the code linked above is relying on undefined behavior. I wish I had looked more closely at the use of block inputs in audiofilters because I would have asked that it be reworked to not rely on a side effect of a synthio.Synthesizer in this way.