json: unstable: initial version

[amirouche]

• [x] chez
• [x] chibi
• [x] gauche
• [x] guile
• [x] sagittarius
• [x] gambit
• [x] gerbil
• [x] loko
• [x] racket
• [x] mit
• [x] chicken
• [ ] cyclone, blocked by https://github.com/justinethier/cyclone/issues/149