Use latest stable Rust CI + Fix Test Errors

[mxgrey]

This PR will test whether using the latest stable version of Rust will result in cause CI to fail as suspected in #419 (comment)

[mxgrey]

Even after #419 was merged, the CI for this PR still showed some clippy errors. I've updated and repurposed this PR to get the CI green for the latest stable version of Rust.

@esteve I just need to check on how you'd like to resolve this comment

I'd rather have an explicit version of the Rust toolchain there (instead of stable)

versus this comment

we should do that here as well, with the latest stable Rust toolchain to catch any issues with the compiler

I've noticed that this line already has the CI run every day, which is probably more frequently than what's really needed, but the daily runs don't cost us anything so 🤷 . Are you okay with using @stable as the PR already does, or do you want me to put in a fixed toolchain version?

For what its worth we do express a minimum supported Rust version in the Cargo.toml of rclrs (which needed to be updated for this PR to pass since we've been using a language feature that stabilized in 1.70). Maybe that's sufficient to express what version we're supporting?