Integrate HRMP (#258)

* HRMP message ingestion

* Plumb hrmp_watermark to build_collation

* Plumb hrmp_watermark to ValidationResult

* Plumb hrmp outbound messages

* Implement message-broker part of HRMP

* Kill UPWARD_MESSAGES as well

Otherwise, they will get resent each block

* Add sudo versions for easier testing

* Remove the xcmp module

Not useful for the moment

* Doc for HRMP message handler

* Estimate the weight upper bound for on_finalize

* Remove a redundant type annotation

* fix spelling of a method

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

* Deabbreviate dmp and hrmp in the message ingestion type

* Don't use binary_search since it's broken by a following rotate

Instead use the linear search. We can afford linear search here since
due to limited scalability of HRMP we can only have at most a couple of
dozens of channels.

* Fix the watermark

Co-authored-by: Bastian Köcher <[email protected]>