Implement basic upward & downward messages (#118)

* Start by replacing branch names and set `DownwardMessage`

* Add the upward-message crate

* Add Kusama & Polkadot

* More work on getting the upward messages working

* Fix build

* Begin to integrate it into the test Parachain

* Update

* Make everything compile again

* Switch to westend and print parachain account on startup

* Use MultiSignature etc

* Fix validate block

* Some downward messages work

* Update git reference

* More downward messages integration

* Update test runtime for downward messages

* Enable downward message handler and withdraw send tokens

* Add some docs

* Begin to implement simple XCMP

* More work

* Fixes and make parachain id configurable

* Make parachain ID be part of the genesis

* Finishing the XCMP message demo

* Update and fixes tests

* Update branch

network/src/tests.rs

@@ -439,6 +439,10 @@ sp_api::mock_impl_runtime_apis! {
 				parent_hash: Default::default(),
 			}
 		}
+
+		fn downward_messages(_: ParaId) -> Vec<polkadot_primitives::DownwardMessage> {
+			Vec::new()
+		}
 	}
 }