Json : Fix String Writer

Tidy-up and correct logic around writing JSON
encoded strings.

related #13832

Change-Id: I1eca33e11dff4457f85a896c02331c1cd9ae1110
Reviewed-on: https://review.haiku-os.org/617
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Implementation of JSON Streaming Parser

Fix warnings related to x86_64 tests' compilation.

Implementation of JSON Streaming Parser

This change will introduce a streaming parser capability to Haiku. The
existing functionality of writing the JSON data to a BMessage in-memory
model is retained. The new parser implements a SAX-style listener based
interface where the listener accepts parse events. Unit tests have been
supplied for the JSON parser as well.