Support incremental parsing

[cgay]

[Inspired by a conversation with "zulu.inoue" on the common-lisp Discord, and their "jzon" CL JSON parser.]

Should have an API for parsing multiple json objects from a stream. Currently we call read-to-end to get the entire thing. I think this is a very simple fix, I just never did it.

zulu is also providing an event-based interface for lower level parsing in case of handling untrusted data that might cause a stack overflow and so on. Might want to look at that.

Relevant: https://github.com/dylan-lang/json/blob/master/parser.dylan#L370