[boost] Moving utility/declval.hpp to type_traits

I was going to move utility/declval.hpp to type_traits, but type_traits has many unmerged changes. Since the contents of declval.hpp are currently the same in master and develop, replaying its history on top of type_traits:master and type_traits:develop should not affect any later merges, but still, I always prefer to do this when master and develop are identical, so thought I'd ask first. John?