Added mono to stereo conversion and panning to audiomixer.Mixer

[relic-se]

Allow mono audiosample sources to play through a stereo audiomixer.Mixer object and add audiomixer.MixerVoice.panning (with synthio.BlockInput support).

These new features greatly improve audiomixer.Mixer usability for sample-based musical instruments. It also optimizes flash storage when stereo audio files are unnecessary when mixed with a stereo source (you'd otherwise have to double the channel count + file size).

Code Demonstration (requires appropriate mono.wav and stereo.wav files):

import board
import audiobusio
import audiocore
import audiomixer
import synthio

wav_stereo = audiocore.WaveFile("/stereo.wav")
wav_mono = audiocore.WaveFile("/mono.wav")
if wav_stereo.channel_count != 2 or wav_mono.channel_count != 1:
    raise ValueError("audio file channel counts are incorrect")

audio = audiobusio.I2SOut(
    bit_clock=board.GP20,
    word_select=board.GP21,
    data=board.GP22,
)

mixer = audiomixer.Mixer(
    voice_count=2,
    channel_count=2,
    sample_rate=wav_stereo.sample_rate,
    bits_per_sample=wav_stereo.bits_per_sample,
)

for i in range(len(mixer.voice)):
    mixer.voice[i].level = 0.5
    mixer.voice[i].panning = synthio.LFO(rate=0.5*(i+1))

audio.play(mixer)

mixer.voice[0].play(wav_stereo, loop=True)
mixer.voice[1].play(wav_mono, loop=True)

Closes #4463

[relic-se]

It looks like a few atmel-samd boards are running out of flash firmware space. Would it be better for me to disable this feature (panning and mono-to-stereo) on those boards or disable a larger existing feature?

The following boards are affected:

• feather_m4_express
• itsybitsy_m4_express
• metro_m4_express
• seeeduino_wio_terminal