Consolidate JSON serialization code

[Chris Hillery]

Paul has recently consolidated some of the JSON module serialization code, in particular adding a single function for escaping JSON strings (zorba::json::serialize(ostream&, char const*)).

We should ensure we are using this code in all places that it makes sense, most specifically in the JSONiq serialization code (src/api/serialization/serializer.cpp, class json_emitter).

If there are other avenues for code sharing (eg., for JSON objects and arrays), they should be taken as well.

[Paul J. Lucas (paul-lucas)]

I already did make the code in src/api/serialization/serializer.cpp share code. I'm not aware of any other serialization code. Hence, reassigning.