Add a dbg! macro like std::dbg! for quick-and-dirty debugging

[KodrAus]

Closes #168

This PR introduces emit::dbg!, a macro like std::dbg! for quick-and-dirty debugging:

let id = 42;
let user = "Rust";

emit::dbg!(user, id);

Instead of capturing values with Display + 'static by default, it captures using Debug. The macro produces a template for you, so the resulting event looks like:

You can also specify your own template if you like, just like you can with emit::debug!:

emit::dbg!("got {user} with {id}", user, id);

The motivations for this macro are the same as std::dbg!; you're actively working on some complex code and just want to see what it's doing. It's not intended for long-lived diagnostics, so should be considered a smell to see it hanging around.

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Still to do:

• UI tests
• Update book
• Update readme