Add NeoSkeleton default length

[sffc]

Any objections?

[robertbastian]

I don't see how Long is more qualified as a default than Medium. Why do we need a default? Does this simplify the usage of the API?

[sffc]

The length is not being used for times and time zones, so this makes call sites a little easier to read. #5246

However, I'm now thinking I should just change the constructors to take an options bag where the length is one of the fields, like we do literally everywhere else, giving us more flexibility in the long run.

[sffc]

However, I'm now thinking I should just change the constructors to take an options bag where the length is one of the fields, like we do literally everywhere else, giving us more flexibility in the long run.

Even if I make this change, I still need a default length, because I can't load data without the length. Alternatively, I can make the constructor return an error if the length is not set in the API.

[sffc]

I guess another way around this is I can add another associated type to the trait system to suppress the need to pass a length into the constructor.

[sffc]

Okay, I tried adding a type parameter for the options bag. The length ends up being used fairly deep in the stack, including during the formatting calls to select the correct pattern. (This avoids a map_project in the constructor.) So, internally, I would likely need to still store an actual SkeletonLength but make it an Option<SkeletonLength> and GIGO if I need to retrieve it.

[sffc]

#5248

If #5248 is approved and merged, I will close this PR.

[sffc]

Made obsolete by #5248