Move headers out of boost/pending

[jeking3]

Headers were likely never supposed to live in boost/pending for a long time. At this point boost/pending should be retired and the headers moved into their rightful and normal place. Here are the pending headers for this repository:

iterator/include/boost/pending/detail/int_iterator.hpp
iterator/include/boost/pending/iterator_adaptors.hpp
iterator/include/boost/pending/iterator_tests.hpp