Make migrations a bit less tedious to write.

Ideas:
1. It seems like many `SchemaUpdate` subclasses have inherent inverses, e.g., it should be easy to derive a `DropTable` from a `CreateTable`. Could we simplify `upgrade()` and `downgrade()` into a single function that returns objects that can process both directions of changes?
2. Maybe add a `CompoundUpdate` or similar that applies multiple updates in a single migration. This might be more complex than it seems though if the migration executor and/or migration status information assume that migrations are atomic.