Opt-in to the legacy behaviour

• In tidyr 1.0.0,the interface of tidyr::nest() and tidyr::unnest() changed. But the old interface remains available via tidyr::nest_legacy() and tidyr::unnest_legacy().

• dplyr 1.1.0 takes advantage of a much faster algorithm for computing groups, but the global option dplyr.legacy_locale allows a user to explicitly request the legacy behaviour.