This commit adds support in the json backend for using an external
library that I wrote called jsonstreams. It's a pretty self-explanatory
library, and for piglit has several advantages. First, I've measured
a consistent 10-15 second speed up for running the quick profile.
Second, it *vastly* reduces the amount of memory piglit needs to write
the final JSON document out.

This is not implemented as a separate backend because the way that
piglit's backend are implemented only one backend can handle a specific
file extension. While this can be worked around by setting the
extension to something like '.junit.xml' that doesn't really make sense
here, since they are writing the same format, just using different
means.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>