Use object-inspect instead of util.inspect for stable json output.

This is a compromise. object-inspect seems to give stable output,
but it isn't as pretty as util.inspect. Perhaps someday we'll
enhance object-inspect so we get the best of both worlds.

For now, we use util.inspect by default for TTY output,
and object-inspect for file output (unit tests).
We can use object-inspect with TTY output by setting the
context variable _jsonStable to true.