Audit datetime format function

[robertbastian]

I've found several bugs:

• FieldLength::TwoDigit truncates the number, which is incorrect. Only the yy and YY patterns require truncation. I've moved this code to the year branch, and renamed the variant Two
• c..cc and e..ee require locale-sensitive day-of-week-number calculations, so they are now rejected during data loading
• FieldLength::{Abbreviated, Wide, Narrow} also apply to numeric fields, where they mean 3, 4, 5. I renamed them and updated documentation.
• Cyclic year names should be loaded from data (#3761)

[sffc]

The names of the FieldLength enum variants predate my time working on this crate. I assume they were chosen to reflect the primary use case for those widths, even if they mean different things in different contexts