Make stable_adapter rely no is_stable.
Make stable_adapter rely no is_stable.

stable_adapter now relies on is_stable rather than on is_always_stable,
which means that it is now able not to use make_stable when a sorter is
stable when called with specific arguments even though it might be
unstable the rest of the time.

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