message: Add member typedef `value_type` to `StatusOr` to be conformant with the
  standard, e.g. `std::optional`, `std::expected`.
diff: @@ -92,7 +92,8 @@ class StatusOr : private internal_statusor::StatusOrData<T>,
  typedef internal_statusor::StatusOrData<T> Base;
  public:
  - typedef T element_type;
  + typedef T element_type; // DEPRECATED: use `value_type`.
  + typedef T value_type;
  // Constructs a new StatusOr with Status::UNKNOWN status. This is marked
  // 'explicit' to try to catch cases like 'return {};', where people think
  