Separate up the audio unit `SubType` variants into specific enums associated with each `Type` variant.

[mitchmindtree]

This could significantly improve the ergonomics and safety of the AudioUnit::new API.

Rather than taking both a Type and SubType, it should take any T: Into<Type>, where:

• Type's variants hold their own sub type data
• From<$subtype> (for every $subtype) is implemented for Type

This way a user can just pass any single type that can be automatically converted to a Type.

[mitchmindtree]

Hmmm... An alternative option might be to instead:

• Turn the Type enum variants into their own traits.
• Turn each SubType variant into its own unique type, implementing the traits that apply to it.

This is probably a little overly complex and would mean an AudioUnit's type would have to be known at compile-time, which, despite being the most common case, may be limiting for others.