In rust-nightly CI, forbid debugging prints.

This patch makes the rust-nightly CI task fail if it detects any dbg!(), println!(), or eprintln!() calls in production code.

Because of clippy limitations, it may also gripe about calls to these macros in our tests. The preferred workarounds are to either instead. Both are acceptable.

We're doing this check in CI rather than unconditionally with clippy directives, since we often want to have these calls in our code temporarily while we're developing. Some day we might want this test to go into a pre-push hook.

This patch also adds #![allow()] directives for println!() and eprintln!() in the arti crate. Since that one isn't a library, it's okay for it to speak to stdout/stderr.

Closes #218 (closed).