Fix two `Debug`

[Philippe-Cholet]

• TakeWhileInclusive debug implementation missed a field ;
• GroupingMapBy was not really debug because functions usually are not ;
• FlattenOk: use our debug macro instead.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (62d2b65) 93.43% compared to head (6c7932d) 93.56%.

Files	Patch %	Lines
src/grouping_map.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #860 +/- ## ========================================== + Coverage 93.43% 93.56% +0.12% ========================================== Files 48 48 Lines 6778 6771 -7 ========================================== + Hits 6333 6335 +2 + Misses 445 436 -9 ```

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