(core) Adding backend for 2-way references

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

app/common/schema.ts

@@ -4,7 +4,7 @@ import { GristObjCode } from "app/plugin/GristData";
 
 // tslint:disable:object-literal-key-quotes
 
-export const SCHEMA_VERSION = 42;
+export const SCHEMA_VERSION = 43;
 
 export const schema = {
 
@@ -41,6 +41,7 @@ export const schema = {
     displayCol          : "Ref:_grist_Tables_column",
     visibleCol          : "Ref:_grist_Tables_column",
     rules               : "RefList:_grist_Tables_column",
+    reverseCol          : "Ref:_grist_Tables_column",
     recalcWhen          : "Int",
     recalcDeps          : "RefList:_grist_Tables_column",
   },
@@ -264,6 +265,7 @@ export interface SchemaTypes {
     displayCol: number;
     visibleCol: number;
     rules: [GristObjCode.List, ...number[]]|null;
+    reverseCol: number;
     recalcWhen: number;
     recalcDeps: [GristObjCode.List, ...number[]]|null;
   };