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/test/prevector_tests.cpp

@@ -5,6 +5,7 @@
 #include <vector>
 #include "prevector.h"
 
+#include "reverse_iterator.h"
 #include "serialize.h"
 #include "streams.h"