Issue: We observed that, when creating a database based on the same
gORM schema multiple times, indexes could appear in different orders,
hurting determinism for use-cases like schema comparison.
In order to fix this, it's simple to switch the ParseIndexes function
to return a list of indices rather than a map, so the callers will
iterate in deterministic order.
* refactor: distinguish between UniqueIndex and Index
* add test
* add ParseIndex test
* modify unique to constraint
* modify unique to constraint
* fix MigrateColumnUnique
* fix test
* fix unit test
* update test mod
* add MigrateColumnUnique to Migrator interface
* fix format lint
* add comment
* go mod tidy
* revert: revert MigrateColumn
* resolve conflicts