JSON implementation

[srush]

A json implementation (even write-only) would be useful and allow for more shiny demos.

This seems like a good template, but maybe needs associative maps (#343):
https://hackage.haskell.org/package/aeson-0.11.3.0/docs/Data-Aeson-Types.html

Some ideas:

• Dataframes
• Vega-Lite / Plotly (https://vega.github.io/vega-lite/)
• three.js models

These overlap a bit with Dex libs, but there was a lot of thought put into things like Vega, not sure it is necessary to rewrite from scratch.

[oxinabox]

It might need associative maps, but it probably wouldn't need hashmaps.
(I should finish #343 at some point though).
Since JSON tends to have small number of keys, and technically speaking one is supposed to preserve order.
So a list-backed dictionary would be fine.

API-wise: It would be really nice if we had something that was like Show but was parameterized by MIMEtype and value type.
(which is what julia's show is).
Then we would define a method for the MIMEtype application/json.
Which could for simple types fallback to a generic method that is like show, and for dictionaries and other types
(this is a not at all subtle request for multiple-dispatch / multi-parameter type classes)

[srush]

That's helpful thanks @oxinabox .

I now realize this probably needs recursive ADTs to do nicely. Although maybe I'll try a JSONShow approach for fun.