Consider mutation testing

[gilch]

100% test coverage doesn't mean that everything is actually tested. I'm not even sure if I've covered all my lines yet. I think the compiler could use a little refactoring, but I'm not quite confident enough in my tests to try it. I'm not sure which mutation testing package to use yet, or how well Travis or the like can use them.

[gilch]

I have at least covered all of my lines now. That only means they were run by the tests, not that the tests actually checked everything (but at least they didn't crash). Mutation testing is a much higher level of assurance. I've used mutmut on Python 2 code before, but there may be faster options for Python 3 now.

[gilch]

I'm also considering full type annotations and CrossHair contracts. That's probably about the best we can do short of rewriting it in Idris and proving theorems.