Use valuable for structured logging

I've been thinking about how we might implement structured logging in Arti, and stumbled upon valuable, which looks very promising. The tracing crate has unstable experimental support for valuable types, so we can't use it just yet, but it's worth keeping an eye out for when it gets stabilized.

See also: https://github.com/tokio-rs/tracing/discussions/1906

The TLDR is, this would enable us to simply derive Valuable for the types that we want to log as structured fields, and automagically have them displayable by tracing in a reasonable format.