tracing vs log

[joshka]

I noticed when reading the code, there's a lot of log::trace!("[calloop] some message) calls.
I wonder if you'd be ok with moving the lib to just use tracing (with a few small changes for structured values being reported).

Some things of note:

• tracing is a dependency of the polling crate, so it will be in the dependency tree already
• tracing is a dependency of the biggest dependant of calloop by downloads (winit)
• Tracing brings in proc-macro2 and syn. I saw you removed thiserror recently with the aim to remove these crates, but these are deps of the polling crate also (due to tracing).
• tracing can write to log if needed.

PR incoming that aligns with this.