Migrate fixtures to neo datetime

[sffc]

For posterity, here is the final head-to-head benchmark before I delete it for the old formatter:

datetime/datetime_components
                        time:   [486.25 µs 489.23 µs 493.44 µs]
Found 14 outliers among 100 measurements (14.00%)
  8 (8.00%) low mild
  2 (2.00%) high mild
  4 (4.00%) high severe

datetime/datetime_lengths
                        time:   [40.602 µs 40.683 µs 40.769 µs]
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high severe

datetime/neoneo/datetime_lengths
                        time:   [34.485 µs 34.574 µs 34.689 µs]
Found 5 outliers among 100 measurements (5.00%)
  5 (5.00%) high severe

I never added the components bag benchmarks to the neo formatter; I will migrate them directly to semantic skeleta.

[sffc]

Obsoleted by other PRs. The latest bench numbers are here: #1317 (comment)