Replace `use id = path;` with `use path as id;`

[Rendered view](https://github.com/pnkfelix/rfcs/blob/fsk-use-path-as-id/active/0000-use-path-as-id.md)
That’s what I had understood. I still think it should probably be changed. (Or even replace `@` with `=`, then the order would make sense.) Anyway, that’s outside the scope of this issue.