REF/PERF: deduplicate kth_smallest
Ensuring a contiguous input seems to give about a 5-10% performance improvement on the existing benchmark `gil.ParallelKth`
@mzeitlin11 any top-level user visible perf worth mentioning in the what's new? (e.g. nlargest / nsmallest) > @mzeitlin11 any top-level user visible perf worth mentioning in the what's new? (e.g. nlargest / nsmallest)

Based on the benchmark should speed up nlargest/nsmallest about 5-10%. Let me know if that's worth adding a whatsnew thanks @mzeitlin11 

ok this was more of a refactor then.