k_smallest variants (2)

[Philippe-Cholet]

Follow-up of the recent #654.

I wanted to only update the documentation as I did not want to burden ejmount with it.
But I missed a few things:

• Function bounds: we usually use FnMut and not Fn.
• izip in the test could truncate the result, replaced by our assert_equal.
• add a "generic test" similar to an existing one.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.57%. Comparing base (6814180) to head (d6ea87a).
Report is 20 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #885      +/-   ##
==========================================
+ Coverage   94.38%   94.57%   +0.18%     
==========================================
  Files          48       48              
  Lines        6665     6749      +84     
==========================================
+ Hits         6291     6383      +92     
+ Misses        374      366       -8     

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[phimuemue]

Thanks.