Introduce message broker for receiving and sending relay chain messages (#80)

* Start message broker implementation

* Finish first stub implementation

* Add features

* Fix attribute

* Update primitives/src/lib.rs

Co-Authored-By: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>

Co-authored-by: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>

primitives/src/lib.rs

@@ -0,0 +1,53 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate 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.
+
+// Substrate 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 Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Cumulus related primitive types and traits.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+/// Identifiers and types related to Cumulus Inherents
+pub mod inherents {
+	use sp_inherents::InherentIdentifier;
+
+	/// Inherent identifier for downward messages.
+	pub const DOWNWARD_MESSAGES_IDENTIFIER: InherentIdentifier = *b"cumdownm";
+
+	/// The type of the inherent downward messages.
+	pub type DownwardMessagesType = Vec<()>;
+}
+
+/// Well known keys for values in the storage.
+pub mod well_known_keys {
+	/// The storage key for the upward messages.
+	///
+	/// The upward messages are stored as SCALE encoded `Vec<()>`.
+	pub const UPWARD_MESSAGES: &'static [u8] = b":cumulus_upward_messages:";
+}
+
+/// Something that should be called when a downward message is received.
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+pub trait DownwardMessageHandler {
+	/// Handle the given downward message.
+	fn handle_downward_message(msg: &());
+}
+
+/// Something that can send upward messages.
+pub trait UpwardMessageSender {
+	/// Send an upward message to the relay chain.
+	///
+	/// Returns an error if sending failed.
+	fn send_upward_message(msg: &()) -> Result<(), ()>;
+}