Specialize `MapForGrouping::fold`

[Philippe-Cholet]

Related to #755

MapForGrouping is only internally used by GroupingMapBy and it only calls for_each on it (which in turn rely on fold). So I allow the clippy lint missing_trait_methods because specialize other methods is simply useless. (In case we run clippy with warning it.)

I could replace next body by unreachable!() and it would still work fine. Anyway, it will disappear from test coverage now that we have one (see here).

I previously wandered how to test and benchmark this:

• The current tests will test this.
• And I guess I could benchmark this fold specialization through some into_grouping_map_by benchmark but I simply don't think it's worth the time: no doubt adapted_iterator.map.fold is faster than the default fold calling next repeatedly. We just don't know how much faster.

Note that all GroupingMapBy methods ultimately rely on this fold so this specialization will improve performance for all its methods.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6814180) 94.38% compared to head (79f76be) 94.15%. Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #873 +/- ## ========================================== - Coverage 94.38% 94.15% -0.24% ========================================== Files 48 48 Lines 6665 6673 +8 ========================================== - Hits 6291 6283 -8 - Misses 374 390 +16 ```

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[phimuemue]

After a second thought, I'm pretty sure type MapForGrouping<I, F> = MapSpecialCase<I, MapForGroupingFn<F>> would be cleaner.

[Philippe-Cholet]

Seems cleaner indeed.