[Feature] make a public api for converting Model to json-ast model (JsonValue)

[stevelr]

I have a use case where I need the json-ast semantic model in memory as a serde_json::Value. JsonWriter computes this Value internally before serializing it out to a json-text representation.

The work-around I'm using is to use JsonWriter to write the model, then use serde_json to read it back in again to construct the Value. If JsonWriter exposed its internal Value available through an api, the in-memory semantic model would be easier to obtain.

One way to implement this would be to expose a new function in the atelier-json crate: pub fn json_ast_model(model: &Model) -> Value

JsonWriter could use this function internally, to avoid duplicating code, and there wouldn't be any api change to JsonWriter.

[johnstonskj]

Working on this now.

• Using name model_to_json for consistency with other names.
• Added all the examples from chapter 17 of the spec as tests.
• Found and fixed a few reader bugs, especially handling traits on members.