Bump the core version

# 0.81.0 Release notes

### API breaking changes:

* `Table::get_parent_row_index()` and `Table::get_index_in_group()` together
  replace `Table::get_index_in_parent()`. This was done to avoid a confusing mix of distinct concepts.

### Enhancements:

* It's now possible to sort a LinkRef according to a column in the target table. Also lets you build a TableView with the sorted result instead. The new methods on LinkViewRef are `sort()` and `get_sorted_view()`