CI: Block warnings (with warnings fixes)

[chrysn]

This adds the equivalent of -Werror to the CI builds.

While this is generally discouraged (for it means that new Rust versions break CI), in this case we don't get new Rust versions all the time but only on riotdocker updates, and that's a good time to reflect on whether the new warnings make sense or not.

Let's see how well that works; I rather add #[allow(...)] generously in the codebase than have so many warnings that important ones get overshadowed.

Along with the actual change, all the fixes that would previously have caused warnings (now errors) come in.