show: Convert JSON format to the new self-recursive style

As before, this is all code movement and a smidgen of glue.  This
moves the existing JSON formatter code into one self-recursive
function, but doesn't change any of the logic to take advantage of the
new structure.

In general, "leafs" of the JSON structure are left in helper functions
(most of them untouched), so that it's easy to see the overall
structure of the format from the main recursive function.