Add meta function to deduce forwarding references.
Add meta function to deduce forwarding references.

This comes in handy when trying to pass the forwarding reference to
another type:

        template <typename T>
        void f(T&& x)
        {
          using freffed = util::deduce<decltype(x), Z>;
          // Work with Z as forwarding reference.
        }

vast-io/vast,vast-io/vast,mavam/vast,pmos69/vast,vast-io/vast,mavam/vast,pmos69/vast,mavam/vast,vast-io/vast,vast-io/vast,pmos69/vast,pmos69/vast,mavam/vast