Injection of non-boost namespaces into global namespace

Like the global namespace injection in include/bind.hpp, this needs to begin being deprecated immediately and removed in a near future Boost as it causes symbol collision between incompatible Boosts:

```
boost/mpl/aux_/adl_barrier.hpp:# define BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE mpl_
boost/mpl/aux_/adl_barrier.hpp:# define BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE_OPEN namespace mpl_ {
boost/type_traits/integral_constant.hpp:namespace mpl_{
boost/type_traits/integral_constant.hpp: using ::mpl_::bool_;
boost/type_traits/integral_constant.hpp: using ::mpl_::integral_c;
boost/type_traits/integral_constant.hpp: using ::mpl_::integral_c_tag;
```