Using with tracing-rs?

Hi :)

I wanted to consider using this for my wip rust mail server. It uses the nowadays fairly common `tracing` crate for logging. However, this crate seems to require a slog logger.

Is there any chance to solve this requirement differently to allow usage with other logging implementations like tracing?

Start by reviewing the crate's current slog logger requirement and how the logging dependency is wired into the Rust implementation. Determine whether support for tracing or other logging implementations can be designed without slog; done means the crate can be used with tracing as requested.

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