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>

cumulus/primitives/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "cumulus-primitives"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+
+[dependencies]
+# Substrate dependencies
+sp-inherents = { git = "https://github.com/paritytech/substrate", branch = "cumulus-branch" }
+
+# Other dependencies
+impl-trait-for-tuples = "0.1.3"
+
+[features]
+default = [ "std" ]
+std = [
+	"sp-inherents/std",
+]