AURA: Switch to CurrentSlot instead of LastTimestamp (#8023)

* Convert AURA to new pallet macro

* AURA: Switch to `CurrentSlot` instead of `LastTimestamp`

This switches AURA to use `CurrentSlot` instead of `LastTimestamp`.

* Add missing file

* Update frame/aura/src/migrations.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Remove the runtime side provide inherent code

* Use correct weight

* Add TODO

* Remove the Inherent from AURA

* 🤦

* Remove unused stuff

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/frame/aura/src/migrations.rs

@@ -0,0 +1,43 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 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.
+
+//! Migrations for the AURA pallet.
+
+use frame_support::{traits::Get, weights::Weight, pallet_prelude::*};
+
+struct __LastTimestamp<T>(sp_std::marker::PhantomData<T>);
+impl<T: RemoveLastTimestamp> frame_support::traits::StorageInstance for __LastTimestamp<T> {
+	fn pallet_prefix() -> &'static str { T::PalletPrefix::get() }
+	const STORAGE_PREFIX: &'static str = "LastTimestamp";
+}
+
+type LastTimestamp<T> = StorageValue<__LastTimestamp<T>, (), ValueQuery>;
+
+pub trait RemoveLastTimestamp: super::Config {
+	type PalletPrefix: Get<&'static str>;
+}
+
+/// Remove the `LastTimestamp` storage value.
+///
+/// This storage value was removed and replaced by `CurrentSlot`. As we only remove this storage
+/// value, it is safe to call this method multiple times.
+///
+/// This migration requires a type `T` that implements [`RemoveLastTimestamp`].
+pub fn remove_last_timestamp<T: RemoveLastTimestamp>() -> Weight {
+	LastTimestamp::<T>::kill();
+	T::DbWeight::get().writes(1)
+}