Creates a new Parsable subclass (of the given parsable_type) from
the given json.
An object of the given parsable_type is created, and its <function>parse_json</function> and <function>
post_parse_json</function> class functions called on the JSON node obtained from json. <function>post_parse_json
</function> is called once on the root node, while <function>parse_json</function> is called for each of the node's
members.
If length is -1, json will be assumed to be nul-terminated.
If an error occurs during parsing, a suitable error from ParserError will be returned.
| parsable_type |
the type of the class represented by the JSON |
| json |
the JSON for just the parsable object |
| length |
the length of |