Manual `Debug` impl for `salsa::Id`

[MichaReiser]

I've often been confused by the fact that the IDs printed by DebugWithDb don't match with the ids printed when dumping a salsa::Id with Debug.

This PR replaces the derived Debug implementation of salsa::Id with a manual Debug implementation that ensures that the IDs printed by Debug and DebugWithdb are consistent.

Open Questions

• Should Id use f.debug_tuple to preserve the name in the Debug output?
• Should DebugWithDb continue to use as_u32 directly or should it just call the Id's Debug implementation (It would change the output if we decide that Debug should output Id(num).

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[nikomatsakis]

* Should `Id` use `f.debug_tuple` to preserve the name in the `Debug` output?

I think yes :)

* Should `DebugWithDb` continue to use `as_u32` directly or should it just call the `Id`'s `Debug` implementation (It would change the output if we decide that `Debug` should output `Id(num)`.

I like that it uses the debug impl, keep that I think.

[MichaReiser]

I updated the Debug implementation to write Id({}).

[MichaReiser]

@nikomatsakis I addressed the feedback. Would you mind taking another look?

[MichaReiser]

Ha, I probably need to start over. @nikomatsakis I don't mind rebasing (re-doing) if this is something we want to merge.

[nikomatsakis]

I'm in favor of this but needs a rebase

[MichaReiser]

I'm in favor of this but needs a rebase

Done