In particular if built with `--all-features` or `features = "all"` we don't
want to accidentally make it look like `YearFlags` are something that people
should use. So: hide it from docs and put it behind a dunder.
It has a ton of great features[1], including stronger statistical signifance
tests, making comparisons to previous or baseline runs, nice plots, and being
able to be run on stable.
1: https://bheisler.github.io/criterion.rs/book/