Introduce src/reverse_iterator.hpp and include it...

...where it will be needed

Taken from https://gist.github.com/arvidsson/7231973 with small
modifications to fit the bitcoin core project

src/reverse_iterator.h

@@ -0,0 +1,39 @@
+// Taken from https://gist.github.com/arvidsson/7231973
+
+#ifndef BITCOIN_REVERSE_ITERATOR_HPP
+#define BITCOIN_REVERSE_ITERATOR_HPP
+
+/**
+ * Template used for reverse iteration in C++11 range-based for loops.
+ * 
+ *   std::vector<int> v = {1, 2, 3, 4, 5};
+ *   for (auto x : reverse_iterate(v))
+ *       std::cout << x << " ";
+ */
+
+template <typename T>
+class reverse_range
+{
+    T &x;
+    
+public:
+    reverse_range(T &x) : x(x) {}
+    
+    auto begin() const -> decltype(this->x.rbegin())
+    {
+        return x.rbegin();
+    }
+    
+    auto end() const -> decltype(this->x.rend())
+    {
+        return x.rend();
+    }
+};
+ 
+template <typename T>
+reverse_range<T> reverse_iterate(T &x)
+{
+    return reverse_range<T>(x);
+}
+
+#endif // BITCOIN_REVERSE_ITERATOR_HPP