Consider relationship between fern and tracing

[daboross]

In the last five years, tracing has become widely popular, and quite useful as an alternative to the log crate.

Look at existing tracing subscribers, especially the standard https://docs.rs/tracing-subscriber/ as well as crates like https://docs.rs/tracing-forest, and see if there's any place fern could still be useful in a tracing world. Even if it's just as a "hey, you use fern 0.6.0/0.7.0, here's a drop-in replacement that also gets you a 1-to-1 configuration of tracing as well as log".

As part of this, regardless of whether we end up implementing a tracing subscriber directly, I would like to add support for log's structured logging.