Simple json printing using existing print framework.

I thought I would have a quick go at implementing a json.dumps printer using the existing print framework.  See discussion in issue #804 for background.

It should print any valid json-able object correctly.  It doesn't handle non-json-able objects correctly, eg numbers as dictionary keys.

I've called the module `_json` so that its only function (dumps) can be used by the proper `json` module to implement a full module.  We can add loads to `_json` in the future.
Yes, true both on account of neatness and puuting certain requirements on compiler. One recent risk is smartass compilers which might throw warnings at such stuff (didn't see that yet, but may anticipate). Anyway this brings us one step closer for (template) metaprogramming, so I'm all on ;-).