ENH: consistent reverse iterator definitions for UList and FixedList

- consistent with C++ STL conventions, the reverse iterators should
  use operator++ to transit the list from rbegin() to rend().

  The previous implementation used raw pointers, which meant that they
  had the opposite behaviour: operator-- to transit from rbegin() to
  rend().

  The updated version only has operator++ defined, thus the compiler
  should catch any possible instances where people were using the old
  (incorrect) versions.

- updated forAllReverseIters() and forAllConstReverseIters() macros to
  be consistent with new implementation and with C++ STL conventions.

applications/test/FixedList/Test-FixedList.C

@@ -38,6 +38,7 @@ See also
 #include "List.H"
 #include "IPstream.H"
 #include "OPstream.H"
+#include <numeric>
 
 using namespace Foam;
 
@@ -48,6 +49,63 @@ int main(int argc, char *argv[])
 {
     argList args(argc, argv);
 
+    if (false)
+    {
+        FixedList<string, 1> ident;
+
+        auto iter = ident.begin();
+
+        Info << iter->size() << endl;
+
+        auto riter = ident.rbegin();
+        Info << riter->size() << endl;
+
+        auto iter2 = ident.rbegin();
+
+        iter2 = iter;
+    }
+
+    {
+        FixedList<label, 15> ident;
+        std::iota(ident.begin(), ident.end(), 0);
+
+        // auto iter = ident.begin();
+        //
+        // iter += 5;
+        // Info << *iter << "< " << endl;
+        // iter -= 2;
+        // Info << *iter << "< " << endl;
+
+        // Don't yet bother with making reverse iterators random access
+        // auto riter = ident.crbegin();
+
+        // riter += 5;
+        // Info << *riter << "< " << endl;
+        // riter += 2;
+        // Info << *riter << "< " << endl;
+
+        Info<<"Ident:";
+        forAllConstIters(ident, iter)
+        {
+            Info<<" " << *iter;
+        }
+        Info<< nl;
+
+        Info<<"reverse:";
+        forAllReverseIters(ident, iter)
+        {
+            Info<<" " << *iter;
+        }
+        Info<< nl;
+
+        Info<<"const reverse:";
+        forAllConstReverseIters(ident, iter)
+        {
+            Info<<" " << *iter;
+        }
+        Info<< nl;
+    }
+
     {
         FixedList<label, 4> list1{1, 2, 3, 4};