Implement HRMP (#1900)

* HRMP: Update the impl guide

* HRMP: Incorporate the channel notifications into the guide

* HRMP: Renaming in the impl guide

* HRMP: Constrain the maximum number of HRMP messages per candidate

This commit addresses the HRMP part of https://github.com/paritytech/polkadot/issues/1869

* XCM: Introduce HRMP related message types

* HRMP: Data structures and plumbing

* HRMP: Configuration

* HRMP: Data layout

* HRMP: Acceptance & Enactment

* HRMP: Test base logic

* Update adder collator

* HRMP: Runtime API for accessing inbound messages

Also, removing some redundant fully-qualified names.

* HRMP: Add diagnostic logging in acceptance criteria

* HRMP: Additional tests

* Self-review fixes

* save test refactorings for the next time

* Missed a return statement.

* a formatting blip

* Add missing logic for appending HRMP digests

* Remove the channel contents vectors which became empty

* Tighten HRMP channel digests invariants.

* Apply suggestions from code review

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Remove a note about sorting for channel id

* Add missing rustdocs to the configuration

* Clarify and update the invariant for HrmpChannelDigests

* Make the onboarding invariant less sloppy

Namely, introduce `Paras::is_valid_para` (in fact, it already is present
in the implementation) and hook up the invariant to that.

Note that this says "within a session" because I don't want to make it
super strict on the session boundary. The logic on the session boundary
should be extremely careful.

* Make `CandidateCheckContext` use T::BlockNumber for hrmp_watermark

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

polkadot/runtime/parachains/src/util.rs

@@ -19,7 +19,6 @@
 
 use sp_runtime::traits::{One, Saturating};
 use primitives::v1::{Id as ParaId, PersistedValidationData, TransientValidationData};
-use sp_std::prelude::*;
 
 use crate::{configuration, paras, router};
 
@@ -34,7 +33,7 @@ pub fn make_persisted_validation_data<T: paras::Trait + router::Trait>(
 	Some(PersistedValidationData {
 		parent_head: <paras::Module<T>>::para_head(&para_id)?,
 		block_number: relay_parent_number,
-		hrmp_mqc_heads: Vec::new(),
+		hrmp_mqc_heads: <router::Module<T>>::hrmp_mqc_heads(para_id),
 		dmq_mqc_head: <router::Module<T>>::dmq_mqc_head(para_id),
 	})
 }