[boost] [core] Move underlying_type to type_traits?

While documenting and cleaning up scoped_enum.hpp I factored out the underlying_type trait to a separate header. Mostly, because I didn't want to #include in scoped_enum.hpp. boost::underlying_type should be equivalent to std::underlying_type, except that it also supports emulated scoped enums. Perhaps boost/core/underlying_type.hpp should be moved to Boost.TypeTraits?