Serialisation/parsing of custom structures in JSON

[robotlolita]

This patch adds traits for types to describe how they can be serialised and reified when using a JSON encoding. The approach here favours control over reification to avoid issues with arbitrary code being able to construct capability-bearing types by simply stating their names; here the person describing the JSON types must provide a mapping for them that describes the lowering and the reification passes.

In order to support custom serialisation, the package forks JSON into standard JSON (json object), and "extended" JSON (extended-json) object. The extended JSON object stores non-standard types as the dictionary {"@type": "unique-tag", "value": ... }, it then uses the mapping contained in the extended JSON type to both serialise and reify these types.

Moving the extended JSON portion to a separate type avoids issues with JSON payloads that may use this convention in a different way, and also discourages a single object with all mappings.

Example usage:

let My-domain = #json-serialisation bare
  | tag: "project" type: #project
  | tag: "package" type: #pkg;
let Json = #extended-json with-serialisation: My-domain;

assert (Json parse: (Json serialise: new project("title", new pkg("name", "filename"))))
  === new project("title", new pkg("name", "filename"));