Use non-spatial audio sinks for music

Since non-spatial rodio sinks appear to be more slightly more performant than spatial sinks, we should use them more in place of spatial sinks. This MR:

• Splits the audio Channel struct into Music and Sfx channels and uses plain Sink's for Music as they don't require a positional component. It does away with channel id's as I don't think we need to use them.
• Updates rodio to 0.10, which includes a newer version of cpal (originally pinned to this commit) and removes the flac and mp3 features to reduce the total dependency count.
• Adds max_sfx_channels as a config setting.
• Fixes #463 (closed)

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