Move AuRa digest from client to primitives (#8245)

* Move AuRa digest from client to primitives

This makes the digest stuff usable from inside the runtime ;)

* Update primitives/runtime/src/generic/digest.rs

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

* Review feedback

* Make BABE use the new functionality

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

substrate/primitives/consensus/aura/src/digests.rs

@@ -0,0 +1,64 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2018-2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Aura (Authority-Round) digests
+//!
+//! This implements the digests for AuRa, to allow the private
+//! `CompatibleDigestItem` trait to appear in public interfaces.
+
+use crate::AURA_ENGINE_ID;
+use sp_runtime::generic::DigestItem;
+use sp_consensus_slots::Slot;
+use codec::{Encode, Codec};
+use sp_std::fmt::Debug;
+
+/// A digest item which is usable with aura consensus.
+pub trait CompatibleDigestItem<Signature>: Sized {
+	/// Construct a digest item which contains a signature on the hash.
+	fn aura_seal(signature: Signature) -> Self;
+
+	/// If this item is an Aura seal, return the signature.
+	fn as_aura_seal(&self) -> Option<Signature>;
+
+	/// Construct a digest item which contains the slot number
+	fn aura_pre_digest(slot: Slot) -> Self;
+
+	/// If this item is an AuRa pre-digest, return the slot number
+	fn as_aura_pre_digest(&self) -> Option<Slot>;
+}
+
+impl<Signature, Hash> CompatibleDigestItem<Signature> for DigestItem<Hash> where
+	Signature: Codec,
+	Hash: Debug + Send + Sync + Eq + Clone + Codec + 'static
+{
+	fn aura_seal(signature: Signature) -> Self {
+		DigestItem::Seal(AURA_ENGINE_ID, signature.encode())
+	}
+
+	fn as_aura_seal(&self) -> Option<Signature> {
+		self.seal_try_to(&AURA_ENGINE_ID)
+	}
+
+	fn aura_pre_digest(slot: Slot) -> Self {
+		DigestItem::PreRuntime(AURA_ENGINE_ID, slot.encode())
+	}
+
+	fn as_aura_pre_digest(&self) -> Option<Slot> {
+		self.pre_runtime_try_to(&AURA_ENGINE_ID)
+	}
+}