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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jswrenn]

Well spotted!