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: b2c9c149


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 <[email protected]>

* Update runtime/common/src/parachains.rs

Co-authored-by: Robert Habermeier <[email protected]>

* Update runtime/common/src/parachains.rs

Co-authored-by: Robert Habermeier <[email protected]>

* Minor fixes

* Fix wasm build

Co-authored-by: Gav Wood <[email protected]>
Co-authored-by: André Silva <[email protected]>
Co-authored-by: Cecile Tonglet <[email protected]>
Co-authored-by: Robert Habermeier <[email protected]>