Symbolic json library support

[pschanely]

Expected vs actual behavior
Because the json module is implemented in C by default, CrossHair cannot analyze it symbolically. (instead, it falls back to testing with a few concrete values)

To Reproduce
This is an example where we'd like to find a counterexample, but cannot.

Ideas for fixing
Cpython actually contains both a pure python implementation and a C implementation. By dropping the appropriate C internal module, we can force use of the pure python version. We did this with the datetime module here, and presumably could do a similar thing for the json module. Note that I'm not completely confident this will be enough to demonstrate the example above, but it'll get us quite a bit closer.

[pschanely]

It's not super performant, but finding counterexamples through JSON encode and decode has been working for a while now! Closing.