codecvt_policy: add BOOST_DEFAULTED_FUNCTION for copy ctor to make it obvious copy ctor is important to use cases.
generic_narrow: add constructor taking CodecvtPolicy argument. Use case isn't clear, but reassuring it will work if needed.
generic_narrow::from_iterator: make it a non-template class. Given the use of std::codecvt, the only valid use case is "const char*", so just hard wire that. KISS.