Is it possible to make 'proc' in json-stream-read take a third, opaque
object, and pass proc's result to the next 'proc' call?
json-stream-read returns the result of the last 'proc' call.

I think by chaining these proc calls together, json-stream-read user
can pass parsing state along and can even avoid mutable states if they
want to. I haven't looked at the implementation though so I don't know
how hard to do it.
--
Duy