Downward Message Processing implementation (#1859)

* DMP: data structures and plumbing

* DMP: Implement DMP logic in the router module

DMP: Integrate DMP parts into the inclusion module

* DMP: Introduce the max size limit for the size of a downward message

* DMP: Runtime API for accessing inbound messages

* OCD

Small clean ups

* DMP: fix the naming of the error

* DMP: add caution about a non-existent recipient

polkadot/runtime/kusama/src/lib.rs

@@ -18,7 +18,7 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 // `construct_runtime!` does a lot of recursion and requires us to increase the limit to 256.
-#![recursion_limit="256"]
+#![recursion_limit = "256"]
 
 use sp_std::prelude::*;
 use sp_core::u32_trait::{_1, _2, _3, _4, _5};
@@ -1134,6 +1134,12 @@ sp_api::impl_runtime_apis! {
 		fn validator_discovery(_: Vec<ValidatorId>) -> Vec<Option<AuthorityDiscoveryId>> {
 			Vec::new()
 		}
+
+		fn dmq_contents(
+			_recipient: Id,
+		) -> Vec<primitives::v1::InboundDownwardMessage<BlockNumber>> {
+			Vec::new()
+		}
 	}
 
 	impl fg_primitives::GrandpaApi<Block> for Runtime {