Factor a low-level json encoding package out of types

[aboodman]

We've been wresting with the ergonomics of the types package (see #141, #54) for a few weeks now.

I think one reason it is hard to make progress is that if we change the shape of the types package, we'll have to change all the callers. And the callers have very different needs. The API you'd want to stream a large amount of structured data into the encoding (e.g., xml_importer) is different than the API you'd want to read and explore a small amount of structure data (e.g., DataStore, or pitchmap/index).

It's possible that we might come up with one general purpose API that does a good job at all needs, but I think it's at least equally possible we'll end up with a few APIs: a streaming read API, a structured read API, a streaming write API, etc.

Anyway, at the very least, getting to a place where it is easy to experiment seems good. So with that in mind, Chris and I talked this morning and decided to factor our a VLLJEP (very low level json encoding package).

The abstraction level we're thinking is basically "It knows where to put the braces". That is, it won't do any structural validation, it won't recurse into references (either during encoding or decoding), it won't know about ChunkStore, etc.

[aboodman]

This is done.