`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[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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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