Reinvestigate `TemporalUnit::Auto` and it's general handling

[nekevss]

Currently, TemporalUnit has Auto as an option. It was raised early on in the implementation (circa Oct. 2023) about whether TemporalUnit::Auto could be removed in favor of Option<TemporalUnit>. The blocker at the time was logic that lived in Duration::round.

With the changes to the specification over the last year, we should reinvestigate whether Auto should be removed.

If it should not be removed, then we should also determine if Auto should be used as the default option for TemporalUnit or if there should be a default implementation at all.

[Manishearth]

One thing to note is that whether or not AUTO is an allowed value depends on the caller of GetTemporalUnitValuedOption. For example, it is allowed if called from GetDifferenceSettings.

With the current code, Instant.since(Instant2) with default options fails, which is probably incorrect.

[Manishearth]

Actually that's just a bug. #316