ska_sort: don't feed a lambda to the pdqsort fallback (#177)
ska_sort: don't feed a lambda to the pdqsort fallback (#177)

The lambda was a leftover from when ska_sort still relied on std::sort
as a fallback. Replacing it with std::less<> + the passed projection
means that pdqsort might use its branchless partition algorithm when it
makes sense.

Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort