[boost] [modularization] Removing iterator->conversion dependency

Hi, in order to remove the dependency from iterator to conversion I suggest to move |boost/implicit_cast.hpp to MPL. | |Vicente | /conversion/ <http://www.pdimov.com/tmp/report-6d1f271/conversion.html> |<boost/implicit_cast.hpp>| * from |<boost/pending/iterator_tests.hpp>| |//boost/implicit_cast.hpp| |template <typename T> inline T implicit_cast (typename mpl::identity<T>::type x) { return x; } |