The cutting block: These features will be removed from itertools eventually (2)

[Philippe-Cholet]

I did some clean up with #878 but I also deprecated some parts recently so... Similarly to #223, I list deprecated parts that will eventually be removed:

• [ ] fn unfold and Unfold deprecated since 0.13.0.
• [ ] Itertools::group_by and GroupBy deprecated since 0.13.0.
• [ ] Remove together:
  • Itertools::fold1 deprecated since 0.10.2, the alternative Iterator::reduce requires MSRV ≥ 1.51.0 ;
  • Itertools::tree_fold1 deprecated since 0.13.0 ;
  • GroupingMap::fold_first deprecated since 0.13.0.
• [ ] fn zip deprecated since 0.10.4, the alternative core::iter::zip requires MSRV ≥ 1.59.0.