* Add Xcm sender.

* Rename XCM sender and add description.

* Update copyright header.

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* update tiny-keccak to 0.2

* update deps except bitvec and shared_memory

* fix some warning after futures upgrade

* remove useless package rename caused by bug in cargo-upgrade

* revert parity-util-mem *

* remove unused import

* cargo update

* remove all renames on parity-scale-codec

* remove the leftovers

* remove unused dep

* 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>

Co-authored-by: Gavin Wood <gavin@parity.io>

Co-authored-by: Gavin Wood <gavin@parity.io>