Set up some test properties for Jawn.

This commit adds testing for parsing strings, rendering to strings,
and testing jvalues for equality.

Future work:
 * file-based tests using tempfiles
 * test async parsing
 * better name than j() for rendering
 * improve rendering speed, a la the work I did on blueeyes.json
 * smarter DeferNum, we should save if it's an int or float
 * better numerical equality
 * possibly support BigInt?

Anyway, that's all for now.