Handling mulaw/alaw

[xd009642]

I was hoping to use ffmpeg::software::resampler to resample some mulaw PCM samples to s16 PCM of a different sample rate. There's no mulaw or alaw in https://docs.rs/ffmpeg/latest/ffmpeg/util/format/sample/enum.Sample.html but then I'm not sure if that's the right place for it if it's just for representing the type in memory