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/primitives/message-dispatch/src/lib.rs

@@ -0,0 +1,49 @@
+// 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/>.
+
+//! A common interface for all Bridge Message Dispatch modules.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+#![warn(missing_docs)]
+
+use bp_runtime::InstanceId;
+
+/// Message dispatch weight.
+pub type Weight = u64;
+
+/// A generic trait to dispatch arbitrary messages delivered over the bridge.
+pub trait MessageDispatch<MessageId> {
+	/// A type of the message to be dispatched.
+	type Message: codec::Decode;
+
+	/// Estimate dispatch weight.
+	///
+	/// This function must: (1) be instant and (2) return correct upper bound
+	/// of dispatch weight.
+	fn dispatch_weight(message: &Self::Message) -> Weight;
+
+	/// Dispatches the message internally.
+	///
+	/// `bridge` indicates instance of deployed bridge where the message came from.
+	///
+	/// `id` is a short unique identifier of the message.
+	///
+	/// If message is `Ok`, then it should be dispatched. If it is `Err`, then it's just
+	/// a sign that some other component has rejected the message even before it has
+	/// reached `dispatch` method (right now this may only be caused if we fail to decode
+	/// the whole message).
+	fn dispatch(bridge: InstanceId, id: MessageId, message: Result<Self::Message, ()>);
+}