Deprecate `GroupingMap::fold_first` for `reduce`

[Philippe-Cholet]

I just became aware of https://github.com/rust-lang/rust/issues/68125 and https://github.com/rust-lang/rust/pull/79805 and the names that were previously considered.

So we have:

• Itertools::fold1 (deprecated) which basically is Iterator::reduce.
• Itertools::tree_fold1 deprecated/renamed tree_reduce in #895 (not merged yet).
• And I now suggest to deprecate GroupingMap::fold_first and rename it reduce (before I finish #901 where I might have wrote a new fold_first_in instead of reduce_in).

What a naming mess!

Hopefully, we might be able to remove all three deprecated methods at the same time and close this page once and for all.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 94.16%. Comparing base (6814180) to head (714ca2e). Report is 31 commits behind head on master.

Files	Patch %	Lines
src/grouping_map.rs	50.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #902 +/- ## ========================================== - Coverage 94.38% 94.16% -0.23% ========================================== Files 48 48 Lines 6665 6755 +90 ========================================== + Hits 6291 6361 +70 - Misses 374 394 +20 ```

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