Resurrect (a minimal version of) message-broker (#234)

This is mostly a copy of the predating version with exception of some
renaming and alterations (e.g. the message handler takes an inbound
downward message by value, not by reference).

cumulus/rococo-parachains/runtime/Cargo.toml

@@ -37,6 +37,7 @@ pallet-transaction-payment = { git = "https://github.com/paritytech/substrate",
 cumulus-runtime = { path = "../../runtime", default-features = false }
 cumulus-parachain-upgrade = { path = "../../parachain-upgrade", default-features = false }
 cumulus-primitives = { path = "../../primitives", default-features = false }
+cumulus-message-broker = { path = "../../message-broker", default-features = false }
 
 # Polkadot dependencies
 polkadot-parachain = { git = "https://github.com/paritytech/polkadot", default-features = false , branch = "master" }
@@ -72,5 +73,6 @@ std = [
 	"rococo-parachain-primitives/std",
 	"cumulus-runtime/std",
 	"cumulus-parachain-upgrade/std",
+	"cumulus-message-broker/std",
 	"cumulus-primitives/std",
 ]

cumulus/rococo-parachains/runtime/src/lib.rs

@@ -200,6 +200,10 @@ impl cumulus_parachain_upgrade::Trait for Runtime {
 	type OnValidationData = ();
 }
 
+impl cumulus_message_broker::Trait for Runtime {
+	type DownwardMessageHandlers = ();
+}
+
 impl parachain_info::Trait for Runtime {}
 
 construct_runtime! {
@@ -214,6 +218,7 @@ construct_runtime! {
 		Sudo: pallet_sudo::{Module, Call, Storage, Config<T>, Event<T>},
 		RandomnessCollectiveFlip: pallet_randomness_collective_flip::{Module, Call, Storage},
 		ParachainUpgrade: cumulus_parachain_upgrade::{Module, Call, Storage, Inherent, Event},
+		MessageBroker: cumulus_message_broker::{Module, Call, Inherent},
 		TransactionPayment: pallet_transaction_payment::{Module, Storage},
 		ParachainInfo: parachain_info::{Module, Storage, Config},
 	}