Run unit tests with Miri in CI

[twilco]

Miri, an interpreter for Rust's mid-level intermediate representation (MIR), can detect certain types of undefined behavior in any Rust code it runs. We should use it when running the Kosmonaut test suite in CI. This should be as easy as:

1. rustup component add miri
2. cargo miri test

Unfortunately, Kosmonaut development and CI environments currently require nightly, and miri seems to often be unavailable in nightly. If/when we switch to stable, we can make this change.