Basic support for JSON

[nobugs-hare]

We have to implement:

• class JSON::Data, effectively a variant which is able to represent all the necessary JSON data types
  • internally, JSON::Data should (a) have one single buffer, AND (b) pointers which are "dual" (either to this single buffer or being an 'owning' pointer).
    • single buffer is created in non-empty constructor of JSON::Data, and in JSON::parse(). As the JSON::Data is modified, use of the single buffer reduces; the buffer is dropped as soon as there are no references to it.
    • this approach allows to avoid multiple allocations in an extremely common scenario of JSON::parse() with subsequent read-only
• two JS-like functions: JSON::stringify(JSON::Data) - returning nodecpp::string, and JSON::parse(const nodecpp::string&), returning (compact) JSON::Data.