Safety: Run tests with miri

[Lorak-mmk]

miri is Rust's built-in undefined behavior detection tool: https://github.com/rust-lang/miri
To increase safety of the project by reducing undefined behavior, we should run our tests with it.

Sadly, it is not possible to use it with FFI, so we can't run our C++ test suite with it. We can however run our Rust tests, which will be a motivation to write more of them, and perhaps port the C++ tests later.