[boost] moving TypeTraits to the boost::type_traits namespace

I think we should finally move TypeTraits to the boost::type_traits namespace. This has only advantages: - clear separation from other boost libraries - the boost namespace is no longer contaminated - when providing additional TypeTraits, it is immediately clear in which namespace they must be implemented These changes would of course be elaborate, but they would bring advantages: consolidation + general review of existing TypeTraits: - are they still necessary / could they be removed? better to use std::TypeTraits - could they be implemented in a more contemporary way? Gero