Proptest-residual-risk: what a passing proptest run tells you

When a proptest test passes, cargo test prints ok, and I kept wondering whether 256 test cases had been enough. So I wrote proptest-residual-risk, which prints three more lines for every passing test: an upper bound, at 95% confidence, on the chance that the next test case fails; how many regions of the code under test the test cases ran; an estimate of the chance that the next test case runs a region none has run. To try it, add it as a dev-dependency and import its proptest! in place of ...