json api WIP patch

Here is a patch for the first part of the JSON API that was recently
discussed. It includes the json parser hook infrastructure and
functions for json_get and friends, plus json_keys.

As is, this exposes the json lexer fully for use by the hook functions.
But I could easily be persuaded that this should be an opaque structure
with some constructor and getter functions - I don't think the hook
functions need or should be able to set anything in the lexer.

Work is proceeding on some of the more advanced functionality discussed.

cheers

andrew