Hide implementation of Value

[cdunn2001]

And I guess the iterators too. Then again, since this will be a major version bump, maybe we should drop the iterators, and later consider the ideas in #288.

This will help with #120 and #6. It will also let us use a "wiping allocator" as for #415 and #152.

[cdunn2001]

I've started an impl branch in my own fork. I'm not sure when I'll time to finish this. Needs several hours of work.

[cdunn2001]

New plan:

• Create Json::Thing as the new API.
• When we are ready,
  • Rename Thing->Value and Value->ValImpl.
  • Move the public classes.

This way, we can develop the new API in parallel with ongoing development.