Let me know if any of the commits in this fork are interesting for you

[JakubKoralewski]

If so which, and if separate PRs: main...JakubKoralewski:refinery:main

I split the ideas into commits so they can be cherry-picked.

Ideas:

1. Add a 'R' type of migration for repeatable migrations (for CREATE OR REPLACE objects like functions and procedures).
2. Reuse code between sync and async versions. Was quite complex, I was surprised in the end I convinced the compiler to accept the code. The constrain() functions somehow worked.
3. fix for a WARN with the 'directory not being a migration' when loading migrations from a directory.
4. Reorder U0 into 0U etc. for alphabetical ordering in file system.