Ensure Just builds and passes tests with minimal versions of dependencies

[casey]

Debian requires that dependent crates be packaged independently. Since the Debian-packaged versions of crates might lag behind the latest version on crates.io, packagers benefit from being able to package older versions of dependencies, since that older version may already be packaged for Debian.

So, in order to give packagers the most flexibility possible when packaging Just, we should make sure that Just actually builds and passes tests with the minimal versions of dependencies, as declared in Cargo.lock.

To ensure this, I'd like to formalize the process of adding and updating dependencies. So far I've done this on an ad-hoc basis, but I think I should actually write a Just recipe that encompasses those steps.

One check that those steps should include is running cargo +nightly generate-lockfile -Z minimal-versions && cargo test, to ensure that just builds and passes all tests with the minimal version of dependencies.

Also consider lowering minimal versions of dependencies if any of them overshoot.