Loosen Fn bounds to FnMut

[Philippe-Cholet]

In #885, I relaxed the Fn bounds to FnMut for the new variants of k_smallest.
It lead me to find out this is also possible for diff_with and Itertools::into_group_map_by.

I'm not sure why we would need Fn bounds in our crate.
clippy does not have a disallowed_traits lint or I would have added Fn to it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.57%. Comparing base (6814180) to head (18721ab).
Report is 24 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #886 +/- ## ========================================== + Coverage 94.38% 94.57% +0.18%  ========================================== Files 48 48 Lines 6665 6750 +85 ========================================== + Hits 6291 6384 +93  + Misses 374 366 -8 

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[jswrenn]

Well spotted!