message: Define Vector::value_type for STL compatibility
  Specifically, this lets the googlemock container matchers work with a
  flatbuffers::Vector.
diff: @@ -257,6 +257,7 @@ template<typename T> class Vector {
  typedef typename IndirectHelper<T>::return_type return_type;
  typedef typename IndirectHelper<T>::mutable_return_type mutable_return_type;
  + typedef return_type value_type;
  return_type Get(uoffset_t i) const {
  FLATBUFFERS_ASSERT(i < size());
  