Adding Bridges code as git subtree. (#2515)

* Add instructions.

* Squashed 'bridges/' content from commit 345e84a21

git-subtree-dir: bridges
git-subtree-split: 345e84a2146b56628e9888c9f5e129cb40e868a9

* Remove bridges workspace file to avoid confusing Cargo.

* Add some bridges primitives to Polkadot workspace.

* Improve docs.

polkadot/bridges/modules/substrate/src/mock.rs

@@ -0,0 +1,117 @@
+// Copyright 2020 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/>.
+
+//! Mock Runtime for Substrate Pallet Testing.
+//!
+//! Includes some useful testing types and functions.
+
+#![cfg(test)]
+// From construct_runtime macro
+#![allow(clippy::from_over_into)]
+
+use crate::{BridgedBlockHash, BridgedBlockNumber, BridgedHeader, Config};
+use bp_runtime::Chain;
+use frame_support::{parameter_types, weights::Weight};
+use sp_runtime::{
+	testing::{Header, H256},
+	traits::{BlakeTwo256, IdentityLookup},
+	Perbill,
+};
+
+pub type AccountId = u64;
+pub type TestHeader = BridgedHeader<TestRuntime>;
+pub type TestNumber = BridgedBlockNumber<TestRuntime>;
+pub type TestHash = BridgedBlockHash<TestRuntime>;
+
+type Block = frame_system::mocking::MockBlock<TestRuntime>;
+type UncheckedExtrinsic = frame_system::mocking::MockUncheckedExtrinsic<TestRuntime>;
+
+use crate as pallet_substrate;
+
+frame_support::construct_runtime! {
+	pub enum TestRuntime where
+		Block = Block,
+		NodeBlock = Block,
+		UncheckedExtrinsic = UncheckedExtrinsic,
+	{
+		System: frame_system::{Module, Call, Config, Storage, Event<T>},
+		Substrate: pallet_substrate::{Module, Call},
+	}
+}
+
+parameter_types! {
+	pub const BlockHashCount: u64 = 250;
+	pub const MaximumBlockWeight: Weight = 1024;
+	pub const MaximumBlockLength: u32 = 2 * 1024;
+	pub const AvailableBlockRatio: Perbill = Perbill::one();
+}
+
+impl frame_system::Config for TestRuntime {
+	type Origin = Origin;
+	type Index = u64;
+	type Call = Call;
+	type BlockNumber = u64;
+	type Hash = H256;
+	type Hashing = BlakeTwo256;
+	type AccountId = AccountId;
+	type Lookup = IdentityLookup<Self::AccountId>;
+	type Header = Header;
+	type Event = ();
+	type BlockHashCount = BlockHashCount;
+	type Version = ();
+	type PalletInfo = PalletInfo;
+	type AccountData = ();
+	type OnNewAccount = ();
+	type OnKilledAccount = ();
+	type BaseCallFilter = ();
+	type SystemWeightInfo = ();
+	type DbWeight = ();
+	type BlockWeights = ();
+	type BlockLength = ();
+	type SS58Prefix = ();
+}
+
+impl Config for TestRuntime {
+	type BridgedChain = TestBridgedChain;
+}
+
+#[derive(Debug)]
+pub struct TestBridgedChain;
+
+impl Chain for TestBridgedChain {
+	type BlockNumber = <TestRuntime as frame_system::Config>::BlockNumber;
+	type Hash = <TestRuntime as frame_system::Config>::Hash;
+	type Hasher = <TestRuntime as frame_system::Config>::Hashing;
+	type Header = <TestRuntime as frame_system::Config>::Header;
+}
+
+pub fn run_test<T>(test: impl FnOnce() -> T) -> T {
+	sp_io::TestExternalities::new(Default::default()).execute_with(test)
+}
+
+pub fn test_header(num: TestNumber) -> TestHeader {
+	// We wrap the call to avoid explicit type annotations in our tests
+	bp_test_utils::test_header(num)
+}
+
+pub fn unfinalized_header(num: u64) -> crate::storage::ImportedHeader<TestHeader> {
+	crate::storage::ImportedHeader {
+		header: test_header(num),
+		requires_justification: false,
+		is_finalized: false,
+		signal_hash: None,
+	}
+}