Clean up JSON code:

This is the beginning of a cleanup of our JSON parsing/generating code. A lot of it is cosmetic, but large chunks of unused code have removed and existing code has been streamlined. 

* Remove obsolete files;
* Remove obsolete preprocessor configuration tags and decorations;
* Remove arcane functionality (YAML compatibility, comment support);
* Enforce strict mode (single root);
* Improved parsing of numerical types;
* Other misc. cleanups and fixups.

Reviews: @vinniefalco, @rec and @ximinez