Deprecate GroupingMap::fold_first for reduce

[Philippe-Cholet]

I just became aware of rust-lang/rust#68125 and rust-lang/rust#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 Deprecate tree_fold1 for tree_reduce #895 (not merged yet).
• And I now suggest to deprecate GroupingMap::fold_first and rename it reduce (before I finish More generic maps (in GroupingMap) #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]

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 ⚠️

Additional details and impacted files

@@            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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