message: Add SortDescriptor::is_ascending()
diff: @@ -189,6 +189,14 @@ public:
  return DescriptorType::Sort;
  }
  + util::Optional<bool> is_ascending(size_t ndx) const
  + {
  + util::Optional<bool> ret;
  + if (ndx < m_ascending.size())
  + ret = m_ascending[ndx];
  + return ret;
  + }
  +
  void merge_with(SortDescriptor&& other);
  Sorter sorter(Table const& table, const IndexPairs& indexes) const override;
  