Add unsafe checking to test suite

[benesch]

In #74, #75, #76, and #77, @icmccorm worked through some potential memory errors with the unsafe code in this repository that were surfaced by an experimental fork of Miri that can detect borrowing violations in foreign functions: https://github.com/icmccorm/miri

It's still a research prototype, but when it becomes suitable for wider use, or integrated with https://github.com/pnkfelix/krabcake, we should investigate running it automatically in CI. Otherwise we're likely to introduce new memory errors during refactorings.