[meta] Support a JSON value type

[rnewman]

It would be convenient for consumers if we supported the JS equivalent of a blob — a JSON object. We would serialize and deserialize automatically, and they wouldn't collide with strings in the store.

[rnewman]

Another observation: this would integrate nicely with pull (#110). One can imagine a JSON-navigating pull syntax extension, perhaps using strings instead of attributes for the existing sub-resource navigation syntax:

[:find (pull ?e [:person/name {:foo/json ["id" "tag"]}])
 :where [?e :person/age ?age] [(> ?age 21)]]

which would fetch, for each person, their name and the id and tag properties of each :foo/json value.

If broken up into several attributes instead of JSON, that would simply be:

[:find (pull ?e [:person/name {:foo/notjson [:foo/id :foo/tag]}])
 :where [?e :person/age ?age] [(> ?age 21)]]

[rnewman]

Pointer to SQLite's extension:

https://www.sqlite.org/json1.html