feat(edit): Expose ImDocument, an Immutable Document

[epage]

The initial inspiration for ImDocument is that you can parse to it to lookup spans.
There is a chance for performance improvements as well because you can parse an ImDocument from a borrowed type the Deserializers are updated to take advantage of that.

Potential future improvements include

• Allow deserializing from a &ImDocument so you don't have to clone the document to both deserialize and look up spans for richer errors
• Allow deserializing borrowed data. This gets complicated because Value doesn't support borrowed data (and switching to owned). We'd either have to change Value or check if a string is equal to the substr the span points to and then deserialize as a borrowed value.

With this, I wonder if we should transition Document to being MutDocument, and ImDocument to being Document

Compatibility: toml_edit::de::Deserializer::new is deprecated

Compatibility: toml_edit::de::Deserializer now has a defaulted generic parameter. Its debatable on whether that is a breaking change because it can't break builds in some corner cases. I viewed the risk low enough to go forward with it.