Added iterator first.

cxxmph/iterator_first.h

@@ -0,0 +1,12 @@
+template <typename iterator>
+struct iterator_first : public iterator {
+  iterator_first(iterator it) : iterator(it) { }
+  const typename iterator::value_type::first_type& operator*() const {
+    return this->iterator::operator*().first;
+  }
+};
+
+template <typename iterator>
+iterator_first<iterator> make_iterator_first(iterator it) {
+  return iterator_first<iterator>(it);
+}