[boost] [declval] non-ascii characters in declval.hpp

Hi Vicente, `utility/declval.hpp` contains a non-ascii character. I tried to fix this and make some improvements. Attached a patch that * removes a non-ascii character, * updates references to the C++11 standard, * fixes obvious oversights (e.g. `// common_type.hpp` --> `// declval.hpp`), * cleans up and reformats the comments (to the style you used in `add_rvalue_reference.hpp`), * renames the include guard macro (`BOOST_TYPE_TRAITS_EXT_...` --> `BOOST_UTILITY_`). May I commit it? Regards, Michel