Add djbsort to cpp_sort

[zamazan4ik]

Hello.
I found one ineteresting sort algorithm. So if you are interested int it, please add it to your library: https://sorting.cr.yp.to/index.html

According to the site, this algo is very fast and abuse inside a lot of AVX2 optimizations (I hope your CPU supports AVX2 too :)

Thank you.

[Morwenn]

Actually there are several SIMD algorithms that I want to add to the library, but to date I haven't been sure how to proceed: one of the plans was to integrate them to the C++17 version of the library, using execution policies and specially providing those alrgorithms with std::par_unseq but I'm not even sure up to which point supporting execution policies is a good idea since several algorithms only make sense in their parallel or SIMD versions; the original design ideas are detailed in issue #22. The fact that execution policies aren't available in either libstdc++ nor libc++ was of course a blocking point.

Another solution was to wait for std::simd but I'll be old by then x)

To be honest, I'm still not quite sure what to do.