internal: tracing improvements and followups

[davidbarsky]

Hi folks! Building on #16394, I've got a few small tweaks:

• Removed the accidental mod.rs usage that I introduced.
• Removed a panic in pat_analysis.rs.
• Recorded the event kind in handle_event to better distinguish what kind of event is being handled.
• Did a small refactor of hprof to have somewhat more linear control flow, and more importantly, write the recorded fields to the output.

The end result is the following:

[davidbarsky]

cc: @matklad, who I know was interested in this type of thing.

[Veykril]

@bors delegate+

[bors]

✌️ @davidbarsky, you can now approve this pull request!

If @Veykril told you to "r=me" after making some further change, please make that change, then do @bors r=@Veykril

[davidbarsky]

@bors r=@Veykril

[bors]

📌 Commit 44494e2 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 44494e2 with merge 0396811...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 0396811 to master...