JSON implementation

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 (https://github.com/google-research/dex-lang/pull/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.

Start by reading the linked Dex PR #343 and the aeson Data-Aeson-Types documentation, paying attention to the discussion of associative maps. Compare the proposal with the listed dataframe, Vega-Lite, Plotly, and three.js use cases. Done means Dex has a JSON implementation, potentially write-only as suggested, that supports a clearly chosen initial scope.

Written by the indexing model from the issue text.