Refactor json_pack()

Refactor json_pack()

* Implement a "scanner" that reads the format string, maintaining state

* Split json_vnpack() to three separate functions for packing objects,
  arrays and simple values. This makes it more clear what is being
  packed, and the object and array structures become more evident.

* Make the skipping of ignored character simpler, i.e. skip ':' and
  ',' independent of their context

This patch shaves around 80 lines of code from the original
implementation.