[boost] promote "unwrap" to ref.hpp

Hi all, boost::reference_wrapper exists at the top-level of boost (in boost/ ref.hpp), but that header provides no way to unwrap a reference_wrapper. There are two implementations floating around of "unwrap", one in boost.bind's implementation details, and one in boost.mpl's implementation details. Is there any problem with providing a version of unwrap in ref.hpp? ron