New attribute for tables that allow deleting

[LHolten]

Foreign keys to tables that allow deleting should have an index. This is important to make deleting efficient. Note that this is already implied by #[no_reference], maybe the new attribute should be #[no_reference(without_index)]

We could rename:

• #[delete]
• #[no_reference] => #[delete(no_reference)]

Either the index is added automatically, or the user needs to add the #[index] attribute. Having the manual annotation makes it more clear which indices exist and can be used.