jsonb: Introduce JsonDom type and parsing/conversion

[blackbeam]

You can now do the following:

fn example_print_jsonb(value: jsonb::Value) {
    let dom = value.parse().expect("error parsing jsonb");
    println!("JSON: {}", serde_json::Value::from(dom));
}

Note that opaque datetime and decimal values are converted to JSON strings (see docs on the JsonDom struct).