Migrate from slog to tracing.

[rcgoodfellow]

Slog itself recommends using tracing, so let's use tracing.

Also: slog env_looger is a bit derpy to set up with defaults that provide useful logging information for falcon CLI applications. In particular, the need to set the environment variable RUST_LOG with std::env::set_var is unsafe from multi-threaded code.

falcon/lib/src/lib.rs

Lines 252 to 261 in 979dc84

	match std::env::var("RUST_LOG") {
	Ok(s) => {
	if s.is_empty() {
	std::env::set_var("RUST_LOG", "info");
	}
	}
	_ => {
	std::env::set_var("RUST_LOG", "info");
	}
	}

Hopefully, we can find a better path forward with tracing for user-configurable log levels with a useful default.

[hawkw]

Let me know if you could use a hand with this one --- always happy to answer tracing-related questions (or go fix anything that's broken)!

[davepacheco]

FWIW, there's a whole RFD about this though it seems to have stalled: https://rfd.shared.oxide.computer/rfd/489

I don't think this by itself is a reason to switch? Programs construct the slog loggers and can specify whatever logger they want, and they can honor whatever environment variables they want in determining that, can't they?