ska_sort

replace std::result_of with std::invoke_result for cpp20

More SFINAE trickery to get this to compile in VS2015

Making the SFINAE slightly simpler so that this compiles in VS2015.

Make the algorithm fall back to American Flag Sort for less than 1024 elements. This was an optimization that I talked about in my blog post that I had accidentally disabled before submitting. I think I disabled it either to debug something or to compare performance between having it turned on and having it turned off. Anyways enabling this optimization again makes the algorithm noticeably faster. Yay :-)

Raising the recursion limit to 16 because this speeds up sorting of log files on my computer.