Expose emplace_back method of underlying std::vector in json_arrayt.

Using this method facilitates construction of the items in the json_array,
using move rather than copy construction.

Author: thomasspriggs

src/util/json.h

@@ -170,6 +170,12 @@ class json_arrayt:public jsont
     return array.back();
   }
 
+  template <typename... argumentst>
+  void emplace_back(argumentst &&... arguments)
+  {
+    array.emplace_back(std::forward<argumentst>(arguments)...);
+  }
+
   typedef jsont value_type; // NOLINT(readability/identifiers)
 };