Using the Map extension

[fitzthum]

I have been using the annotated_evidence field of the Appraisal, but since it's ctually be an extension, I am thinking about creating my own extension to serve a similar purpose. Where the annotated-evidence takes BTreeMap<String, RawValue>, the map extension looks like Map(Vec<(ExtensionValue, ExtensionValue)>).

This seems sub-optimal. First, it's debatable whether this is a map at all given that it allows duplicate values. More importantly, though, I want to put a somewhat deeply-nested struct into my extension. Ideally I wouldn't have to parse the whole thing into extension types. Any thoughts about changing the extensions to use the existing value structs (or better yet, the ones from the json crate)?