json: add sub-section on default de/enconding (staging area)

Cover deconding from &str, default decoding and encoding into String (ref. #86)

(Similar to the previous PR, with added encoding)
> Do you usually use `MemWriter`s when dealing with json encoding? I have never used `MemWriter`s.

No, I haven't used them before either, it was just for the purpose of pretty-printing, but I reckon that's not so crucial...