[FRAME] Make core-fellowship ans salary work for swapped members (#3156)

Fixup for https://github.com/paritytech/polkadot-sdk/pull/2587 to make
the `core-fellowship` crate work with swapped members.

Adds a `MemberSwappedHandler` to the `ranked-collective` pallet that are
implemented by `core-fellowship+salary`.
There is are exhaustive tests
[here](https://github.com/paritytech/polkadot-sdk/blob/72aa7ac17a0e5b16faab5d2992aa2db2e01b05d0/substrate/frame/core-fellowship/src/tests/integration.rs#L338)
and
[here](https://github.com/paritytech/polkadot-sdk/blob/ab3cdb05a5ebc1ff841f8dda67edef0ea40bbba5/substrate/frame/salary/src/tests/integration.rs#L224)
to check that adding member `1` is equivalent to adding member `0` and
then swapping.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/core-fellowship/src/tests/mod.rs

@@ -0,0 +1,23 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#![cfg(test)]
+
+//! Tests for this crate.
+
+pub(crate) mod integration;
+pub(crate) mod unit;