Downward & Upward messages (#1266)

* Downward messages, the front-end.

* Move types around to make them accessible from Parachains

* Fix compilation

* Fix branch

* Make it compile for Cumulus

* Update the branch names

* Add default generic parameter

* Implement `Partialeq`

* Move upward messages into the `ValidationResult`

* Support disabling of the runtime api

* Update branch

* Adds support for handling downward messages

* Implement sending XCMP messages as up/downward messages

* service: update to latest ServiceBuilder changes

* Make it compile

* Initial commit

Forked at: ef2aa428d7
Parent branch: origin/master

* Update substrate branch to cecton-update-polkadot-substrate

* Update substrate & polkadot to cumulus-branch

* Reset branch

* Update primitives/src/parachain.rs

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Update runtime/common/src/parachains.rs

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Update runtime/common/src/parachains.rs

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Minor fixes

* Fix wasm build

Co-authored-by: Gav Wood <gavin@parity.io>
Co-authored-by: André Silva <andre.beat@gmail.com>
Co-authored-by: Cecile Tonglet <cecile.tonglet@cecton.com>
Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

polkadot/validation/src/error.rs

@@ -81,7 +81,10 @@ pub enum Error {
 	/// Block data is too big
 	#[display(fmt = "Block data is too big (maximum allowed size: {}, actual size: {})", size, max_size)]
 	BlockDataTooBig { size: u64, max_size: u64 },
-	Join(tokio::task::JoinError)
+	Join(tokio::task::JoinError),
+	/// Could not cover fee for an operation e.g. for sending `UpwardMessage`.
+	#[display(fmt = "Parachain could not cover fee for an operation e.g. for sending an `UpwardMessage`.")]
+	CouldNotCoverFee,
 }
 
 impl std::error::Error for Error {