## Related
* https://github.com/emilk/egui/pull/5851
* https://github.com/emilk/egui/pull/7988
## What
We want to make it easier to understand the lineage of a `Id` (which is
the parent `Id`, and the parent of that, etc?)
As a first step of that, we want to clarify the different between a
globally unique `Id`, and an `IdSalt`
I also introduced the `AsId` and `AsIdSalt` traits, which are
implemented of anything that implements `Hash` and `Debug`. The `Debug`
half of that is unused here, but will later be used in `Debug` builds to
produce a proper tree.
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