It turns out my test was backwards: it fed the json text into a string
object then pulled it out, which of course round-tripped correctly. Now
things do round-trip correctly, but I had to ignore white-space
differences and edit the numbers as text->double->text doesn't
necessarily round-trip well (though double->text->double does with 17
digits).
It seemed like a good idea since vulkan and gltf resources use JSON.
However, plist and json parsing and writing are separate: there's no
auto-detection for parsing, and the appropriate function must be used
for writing (though reading one then writing the other will work, but
may result in some information loss, or even invalid json (binary)).
Escape characters aren't handled quite to spec yet (eg, no \uxxxx).
The tests are pretty lame, but they're taken from rfc7159 and round-trip
correctly (which is a surprise for the fp numbers).
