Mutation testing

[DavidVorick]

https://en.wikipedia.org/wiki/Mutation_testing

It seems like there's at least one toolset for helping with this in go.

This is something we don't need to do right away, but should probably do before the 1.0. The basic idea is that we intentionally introduce mutations into the code (chainging '>' to '<' or += 1 to -= 1, or adding 15 instead of 16, etc, and that each mutation should cause a test to fail. If it doesn't, it means your test coverage isn't sufficient and doesn't catch simple programmer errors.