Fix potential small issues in stable_adapter
Fix potential small issues in stable_adapter

stable_compare was calling std::forwarf twice on its parameters, and a
few other functions were calling std::move instead of std::forward on
forwardinf references. It probably hadn't caused any issue so far, but
those issues are still best fixed.

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