Change stable_adapter<default_sorter>::type to the class itself
Change stable_adapter<default_sorter>::type to the class itself

It as causing some weird issue where merge_sorter (the base class) was
apparently not constructible from stable_adapter<default_sorter>, yet we
want sorter_adater<T>::type to be constructible from T, so the target of
::type had to be changed. It's probably a minor breaking change but it
doesn't matter much since the new one is actually correct, and
default_sorter is deprecated anyway.

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