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/core-primitives/src/lib.rs

@@ -102,6 +102,26 @@ pub struct InboundDownwardMessage<BlockNumber = crate::BlockNumber> {
 	pub msg: DownwardMessage,
 }
 
+/// An HRMP message seen from the perspective of a recipient.
+#[derive(codec::Encode, codec::Decode, Clone, sp_runtime::RuntimeDebug, PartialEq)]
+pub struct InboundHrmpMessage<BlockNumber = crate::BlockNumber> {
+	/// The block number at which this message was sent.
+	/// Specifically, it is the block number at which the candidate that sends this message was
+	/// enacted.
+	pub sent_at: BlockNumber,
+	/// The message payload.
+	pub data: sp_std::vec::Vec<u8>,
+}
+
+/// An HRMP message seen from the perspective of a sender.
+#[derive(codec::Encode, codec::Decode, Clone, sp_runtime::RuntimeDebug, PartialEq, Eq, Hash)]
+pub struct OutboundHrmpMessage<Id> {
+	/// The para that will get this message in its downward message queue.
+	pub recipient: Id,
+	/// The message payload.
+	pub data: sp_std::vec::Vec<u8>,
+}
+
 /// V1 primitives.
 pub mod v1 {
 	pub use super::*;