message: prevector: add C++11-like data() method
  This returns a pointer to the beginning of the vector's data.
  backports bitcoin@47314e6daad8157c9e36af90a47b3ae7fa0587de
diff: @@ -475,6 +475,14 @@ public:
  return ((size_t)(sizeof(T))) * _union.capacity;
  }
  }
  +
  + value_type* data() noexcept {
  + return item_ptr(0);
  + }
  +
  + const value_type* data() const {
  + return item_ptr(0);
  + }
  };
  #pragma pack(pop)
  