rewrite mudabox to use JObject semantics for serialization

[bmcfee]

Currently, muda.save is a destructive operation: when the jam is serialized, we first pop out the audio buffer jam.sandbox.muda['y'] to avoid serializing the audio. This is obviously bad.

JObject serialization already skips fields that start with _.

A simple fix here would be the following:

• Modify jam_pack to construct muda as a JObject (or Sandbox) rather than a dict
• Push the audio content down a level, ie, muda._audio['y'] and muda._audio['sr']
• Modify all deformation logic accordingly

This way, muda._audio will be skipped during serialization, and save can be a non-destructive operation.