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/xcm/src/v0/mod.rs

@@ -158,6 +158,51 @@ pub enum Xcm {
 	///
 	/// Errors:
 	RelayedFrom { superorigin: MultiLocation, inner: Box<VersionedXcm> },
+
+	/// A message to notify about a new incoming HRMP channel. This message is meant to be sent by the
+	/// relay-chain to a para.
+	///
+	/// - `sender`: The sender in the to-be opened channel. Also, the initiator of the channel opening.
+	/// - `max_message_size`: The maximum size of a message proposed by the sender.
+	/// - `max_capacity`: The maximum number of messages that can be queued in the channel.
+	///
+	/// Safety: The message should originate directly from the relay-chain.
+	///
+	/// Kind: *System Notification*
+	HrmpNewChannelOpenRequest {
+		#[codec(compact)] sender: u32,
+		#[codec(compact)] max_message_size: u32,
+		#[codec(compact)] max_capacity: u32,
+	},
+
+	/// A message to notify about that a previously sent open channel request has been accepted by
+	/// the recipient. That means that the channel will be opened during the next relay-chain session
+	/// change. This message is meant to be sent by the relay-chain to a para.
+	///
+	/// Safety: The message should originate directly from the relay-chain.
+	///
+	/// Kind: *System Notification*
+	///
+	/// Errors:
+	HrmpChannelAccepted {
+		#[codec(compact)] recipient: u32,
+	},
+
+	/// A message to notify that the other party in an open channel decided to close it. In particular,
+	/// `inititator` is going to close the channel opened from `sender` to the `recipient`. The close
+	/// will be enacted at the next relay-chain session change. This message is meant to be sent by
+	/// the relay-chain to a para.
+	///
+	/// Safety: The message should originate directly from the relay-chain.
+	///
+	/// Kind: *System Notification*
+	///
+	/// Errors:
+	HrmpChannelClosing {
+		#[codec(compact)] initiator: u32,
+		#[codec(compact)] sender: u32,
+		#[codec(compact)] recipient: u32,
+	},
 }
 
 impl From<Xcm> for VersionedXcm {