Summary:
Adding support for 2-way references in data engine.
- Columns have an `reverseCol` field, which says "this is a reverse of the given column, update me when that one changes".
- At the time of setting `reverseCol`, we ensure that it's symmetrical to make a 2-way reference.
- Elsewhere we just implement syncing in one direction:
- When `reverseCol` is present, user code is generated with a type like `grist.ReferenceList("Tasks", reverse_of="Assignee")`
- On updating a ref column, we use `prepare_new_values()` method to generate corresponding updates to any column that's a reverse of it.
- The `prepare_new_values()` approach is extended to support this.
- We don't add (or remove) any mappings between rows, and rely on existing mappings (in a ref column's `_relation`) to create reverse updates.
NOTE This is polished version of https://phab.getgrist.com/D4307 with tests and 3 bug fixes
- Column transformation didn't work when transforming RefList to Ref, the reverse column became out of sync
- Tables with reverse columns couldn't be removed
- Setting json arrays to RefList didn't work if arrays contained other things besides ints
Those fixes are covered by new tests.
Test Plan: New tests
Reviewers: georgegevoian, paulfitz, dsagal
Reviewed By: georgegevoian, paulfitz
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D4322
