Add utilities for std::reference_wrapper

This adds the following helpers for handling std::reference_wrapper:

• A predicate IsReferenceWrapper_v<R> for detecting if R is a std::reference_wrapper<T>.
• A helper function resolveRef for resolving references wrapped in std::reference_wrapper. For type T providing a method T::foo() this allows to transparently use resolveRef(t).foo() if t is either a plain T or a std::reference_wrapper<T>.
• A type alias ResolveRef_t<T> to determine the resolved type for T.