Refactor as_sub to make things clearer. (#6503)

* Refactor as_sub to make things clearer.

- `as_sub` becomes `as_alternative`
- `as_sub_limited` becomes `as_derivative`
- `as_alternative` and `as_derivative` generate a mutually exclusive set of accounts.

* Test fix

* Add test

* Fix test

* Remove `as_alternative`.

* Docs.