Merge: json::serialization: intro services to easily serialize to and from Json

Merge: json::serialization: intro services to easily serialize to and from Json

The services `to_json_string` and `from_json_string` should be useful for simple scripts. Both services report errors only on the console. This behavior is enough for simple scripts but more complex programs should still use `Serializer` and `Deserializer` services.

This methods may look a lot like `to_json` and `to_plain_json`. I plan to replace `to_json` with either `to_plain_json` to preserve the same behavior but with support for all serializable classes, or `to_json_string` to get JSON with metadata. Note that even if the output contains metadata, it is readable by external tools with a small overhead.

If you have a better idea for the names, please mention it. For my part, I consider these names temporary until we free the `to_json` names...

Pull-Request: #1822
Reviewed-by: Jean Privat <jean@pryen.org>