Feedback needed: Up/down hooks to insert custom Rust code API

[cljoly]

In #28, @matze implemented hooks, to run Rust code along the SQL statements during a migration.

Here is the new methodes introduced: down_with_hook, up_with_hook

Did you notice any pain points or potential improvements while using it?
Please feel free to comment below, we need your feedback to improve the API before it is stable.