JSON normalizers returns object instead of associative arrays

Json normalizers returns object of stdClass instead of associative arrays
- therefore revisit the serializes in general and simplify them !

benefit:
- distinguish key-value stores from lists: lists/sets are arrays, key/value-stores are objects
- this will help typing a lot !

Start by locating the JSON normalizers and the serializers mentioned in the issue, then inspect how their outputs distinguish objects from lists. Done means the normalizers return associative arrays where appropriate, while lists and sets remain arrays; the broader serializer simplification still needs definition.

Written by the indexing model from the issue text.