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.

Author: gavofyork

src/tests.rs

@@ -201,19 +201,11 @@ fn filtering_works() {
 		assert_ok!(Proxy::proxy(Origin::signed(4), 1, None, call.clone()));
 		expect_event(RawEvent::ProxyExecuted(Err(DispatchError::BadOrigin)));
 
-		let sub_id = Utility::sub_account_id(1, 0);
-		Balances::mutate_account(&sub_id, |a| a.free = 1000);
+		let derivative_id = Utility::derivative_account_id(1, 0);
+		Balances::mutate_account(&derivative_id, |a| a.free = 1000);
 		let inner = Box::new(Call::Balances(BalancesCall::transfer(6, 1)));
 
-		let call = Box::new(Call::Utility(UtilityCall::as_sub(0, inner.clone())));
-		assert_ok!(Proxy::proxy(Origin::signed(2), 1, None, call.clone()));
-		expect_event(RawEvent::ProxyExecuted(Ok(())));
-		assert_ok!(Proxy::proxy(Origin::signed(3), 1, None, call.clone()));
-		expect_event(RawEvent::ProxyExecuted(Err(DispatchError::BadOrigin)));
-		assert_ok!(Proxy::proxy(Origin::signed(4), 1, None, call.clone()));
-		expect_event(RawEvent::ProxyExecuted(Ok(())));
-
-		let call = Box::new(Call::Utility(UtilityCall::as_limited_sub(0, inner.clone())));
+		let call = Box::new(Call::Utility(UtilityCall::as_derivative(0, inner.clone())));
 		assert_ok!(Proxy::proxy(Origin::signed(2), 1, None, call.clone()));
 		expect_event(RawEvent::ProxyExecuted(Ok(())));
 		assert_ok!(Proxy::proxy(Origin::signed(3), 1, None, call.clone()));