Store hash of migration contents when migration is applied

[gonzus]

This is done in postgres-migrations and seems to be a good idea.

[KyleJune]

I thought about that at the time that I made this package but felt it best to leave that functionality out as warnings would cause confusion if someone were to apply new formatting rules to their existing migration files, then there would need to be a way to resolve those.

I feel like the issue of someone change a past migration to behave differently than it use to is something that should be caught in a code review or by test coverage. I agree generally that people shouldn't be modifying the behavior of past migration files as that could lead to new databases differing from existing ones that ran the previous version of the migration file.

[gonzus]

Ok, I see. Of course, please feel free to close this issue. Cheers!