Public issue https://github.com/mackron/miniaudio/issues/323

This can be used for echo as well as a simple delay.

This uses https://github.com/blastbay/verblib to achieve the effect.

  * Biquad
  * LPF
  * HPF
  * BPF
  * Notch
  * Peak
  * Low / High Shelf

This is in preparation for some work in the high level API.

Public issue https://github.com/mackron/miniaudio/issues/101

Public issue https://github.com/mackron/miniaudio/issues/320

Public issue https://github.com/mackron/miniaudio/issues/319

This was happening specifically when uninitializing a data buffer while
it's still loading asynchronously.

This properly decouples data buffer nodes from data buffers and should
address some subtle bugs, especially with uninitializing data buffers
while they're still loading.

This also cleans up the decoding code, in particular by avoiding some
duplicate code between the synchronous and asynchronous decoding paths.