Rework the file layout and implementation of 'is_permutation' to match the various versions of the C++ standard. In particular, move the four iterator versions into the cxx14/ directory.

test/is_permutation_test1.cpp

@@ -11,6 +11,7 @@
 
 #include <boost/config.hpp>
 #include <boost/algorithm/cxx11/is_permutation.hpp>
+#include <boost/algorithm/cxx14/is_permutation.hpp>
 
 #define BOOST_TEST_MAIN
 #include <boost/test/unit_test.hpp>