trace: Support setting event parent events explicitly about tracing HOT 8 CLOSED

I've tried to provide a thorough description of how I would recommend making this change, so it should be appropriate for a new contributor. With that said, if anyone's interested in working on this one and anything I said above is confusing or unclear, please let me know and I'm happy to explain further.

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@hawkw May be wrong on that, but it seems to me that in order to implement Event::child_of(...) the same way as Span::child_of ,the AsId type needs to be moved from tokio-trace to tokio-trace-core in order to be visible there. Correct me if I am wrong :)

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@zaharidichev that's a good point, thanks --- I think I overlooked that. I'd actually like to look into removing the AsId trait in the upcoming release, as I think it can be replaced with Into implementations. So, that may be the best solution.

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Wouldn’t that introduce a breaking change though ?

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Yes. There are other unavoidable breaking changes scheduled for a tokio-trace 0.2 release, though, so now would be a good time to make that change as well.

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@zaharidichev see also #1102 :)

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Great, I already started on this one and will rebase once #1102 is merged. Will be able to pick it back up in the following days though. Someone might beat me to it by then :)

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This was closed by tokio-rs/tokio#1109.

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