Add support for Reclass reference interpolation

[simu]

This PR introduces methods Mapping::interpolate() and Value::interpolate(), which use Token::parse() to parse Reclass references, and then call Token::render() to interpolate the references.

Additionally, the PR introduces methods Value::rendered(), Value::render() and Value::render_with_self() which users should use when wanting to interpolate Reclass references and flatten ValueLists in a Value.

The PR also renames method Node::flatten_parameters() to Node::render_parameters() and extends the method to also interpolate all references by calling Value::render_with_self() on the node's parameters.

Internally, the PR introduces a method Value::raw_string() (see 3c7e449) which produces a stringified version of the Value which is suitable for looking up Reclass reference values and can be used to emit complex values (Mappings and Sequences) for references which are embedded in strings (e.g. using a reference in a multi-line string which is used as a YAML or JSON config file). Complex values are serialized as JSON by the function, while simple values are serialized in a format that matches Python's str().

Additionally, we restructure the refs module to better match the structure of the implementation, see a42b0a0, and introduce a new method Token::parse() (cf. 236c238) to parse an arbitrary string which may contain Reclass references into a Token.

Checklist

• The PR has a meaningful title. The title will be used to auto generate the changelog
• PR contains a single logical change (to build a better changelog).
• Update the documentation.
• Update tests.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency, internal
  as they show up in the changelog

[bastjan]

LGTM! Took a while to understand the interpolate and resolve part.