Adding entry_ids to assocs built with `cast_assoc`

If we apply the `cast_assoc` function to a changeset, the association is added to the changes, and will be inserted into the database if it doesn't exist.

This can cause a problem for us, as the association will be inserted without an entry id.

To fix this, when adding the `entry_id` on the insert function, we should iterate through the changes, checking if there are any nested changesets, and add `entry_id`s to those too.