Multiple minor structural changes to the json parser/generator.

 - Moved the json helper that produces asl structures to the main json header.
 - Wrapped `json.hpp` into the `adobe` namespace.
 - Included two helper functions for the ASL helper to streamline parsing and generation of JSON.
 - renamed `json_parser_t` to `json_parser` in keeping with ASL convention.
 - Updated test cases.
 - `json_generator<T>::generate` now returns its output iterator in keeping with ASL convention.