Merge: Json Ad-Hoc string parser

Simple ad-hoc JSON parser working in a similar way as `DOMXmlParser` or `SExpParser`, on the simple  example, the runtime now is around 1.5 seconds.

Once #1885 is merged however, the runtime is ~0.65 seconds, which is nice.

Depends on #1884

Pull-Request: #1886
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>