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/relays/polkadot-client/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "relay-polkadot-client"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+
+[dependencies]
+codec = { package = "parity-scale-codec", version = "2.0.0" }
+headers-relay = { path = "../headers-relay" }
+relay-substrate-client = { path = "../substrate-client" }
+relay-utils = { path = "../utils" }
+
+# Bridge dependencies
+
+bp-polkadot = { path = "../../primitives/polkadot" }
+
+# Substrate Dependencies
+
+frame-system = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+frame-support = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+pallet-transaction-payment = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+sp-core = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+sp-keyring = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+sp-runtime = { git = "https://github.com/paritytech/substrate.git", branch = "master" }

polkadot/bridges/relays/polkadot-client/src/lib.rs

@@ -0,0 +1,47 @@
+// Copyright 2019-2020 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/>.
+
+//! Types used to connect to the Polkadot chain.
+
+use relay_substrate_client::{Chain, ChainBase};
+use std::time::Duration;
+
+/// Polkadot header id.
+pub type HeaderId = relay_utils::HeaderId<bp_polkadot::Hash, bp_polkadot::BlockNumber>;
+
+/// Polkadot chain definition
+#[derive(Debug, Clone, Copy)]
+pub struct Polkadot;
+
+impl ChainBase for Polkadot {
+	type BlockNumber = bp_polkadot::BlockNumber;
+	type Hash = bp_polkadot::Hash;
+	type Hasher = bp_polkadot::Hasher;
+	type Header = bp_polkadot::Header;
+}
+
+impl Chain for Polkadot {
+	const NAME: &'static str = "Polkadot";
+	const AVERAGE_BLOCK_INTERVAL: Duration = Duration::from_secs(6);
+
+	type AccountId = bp_polkadot::AccountId;
+	type Index = bp_polkadot::Nonce;
+	type SignedBlock = bp_polkadot::SignedBlock;
+	type Call = ();
+}
+
+/// Polkadot header type used in headers sync.
+pub type SyncHeader = relay_substrate_client::SyncHeader<bp_polkadot::Header>;