Optional preserve_order feature to preserve order of vorbis comments (tags)

[asnare]

This PR adds support for a preserve_order feature: when enabled the order of tags is preserved within the VorbisComments block. Semantics are:

• When loading, the order is preserved¹.
• When setting tags, the order will be the order in they were set¹.
• Removal leaves remaining tags in their existing order.
• Replacing a tag leaves it in its existing position.

These semantics are intended to match those of the preserve_order feature available on serde_json.

To preserve compatibility this feature is not part of the default features: the public interface changes when it is enabled.

Footnotes

1. Due to the internal implementation, tags are grouped by key: their position will be the location of the first key, and the values will retain their original order. ↩ ↩²