Use tracing-tree for logging / clean up debug output

[nathanwhit]

This PR makes it so we use the tracing-tree crate for logging output. This adds indentation to the logs, and generally makes the logging output much more readable compared to the current state.

I also cleaned up many of the logging calls to properly use the tracing macros (e.g. create a new span when entering a function instead of just emitting an event).

[jackh726]

This can be merged with a rebase @nathanwhit

[nikomatsakis]

Woah, stuff like debug!(?foo) is surprising to me. Where can I read up on that?

[nikomatsakis]

That said, 👍 to merging, I trust this makes sense even if I don't get it yet

[nathanwhit]

Woah, stuff like debug!(?foo) is surprising to me. Where can I read up on that?

It definitely does take a bit of getting used to, the tracing docs are quite helpful in explaining it. The macro syntax specifically is covered here.

(As a side note, I've been meaning to add some documentation in the chalk book about using the macros, it seems like it would be good to have some of the basics covered there with a link to the full docs.)