Introduce bridge relayers pallet (#1513)

* introduce relayers pallet

* add MessageDeliveryAndDispatchPaymentAdapter

* plug in pallet into test runtimes

* tests prototype

* tests for the relayers pallet

* tests for payment adapter

* mint_reward_payment_procedure_actually_mints_tokens

* benchmarks

* remove irrelevant todo

* remove redundant clone

bridges/primitives/relayers/src/lib.rs

@@ -0,0 +1,45 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Primitives of messages module.
+
+#![warn(missing_docs)]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use sp_std::{fmt::Debug, marker::PhantomData};
+
+/// Reward payment procedure.
+pub trait PaymentProcedure<Relayer, Reward> {
+	/// Error that may be returned by the procedure.
+	type Error: Debug;
+
+	/// Pay reward to the relayer.
+	fn pay_reward(relayer: &Relayer, reward: Reward) -> Result<(), Self::Error>;
+}
+
+/// Reward payment procedure that is simply minting given amount of tokens.
+pub struct MintReward<T, Relayer>(PhantomData<(T, Relayer)>);
+
+impl<T, Relayer> PaymentProcedure<Relayer, T::Balance> for MintReward<T, Relayer>
+where
+	T: frame_support::traits::fungible::Mutate<Relayer>,
+{
+	type Error = sp_runtime::DispatchError;
+
+	fn pay_reward(relayer: &Relayer, reward: T::Balance) -> Result<(), Self::Error> {
+		T::mint_into(relayer, reward)
+	}
+}