Mock XCM (#876)

* sketch downward messages

* bring in attempt to mock mqc-head from moonbeam

* just patch individual crates

* fing comma

* add some logs

* Holy shit, we actually imported a block!

* Actually mock the message queue chain

* use relay parent number for `sent_at`

* finish moving MQC to primitives

* more complete mock and better config type

* change name

* fix export

* better map types

* fix dependencies after rebase

* try-rejigging branches because this is an override

* try to re-jig for hrmp mcqs

* fix branches

* actually fix branches better

* even better

* Removestray log lines

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Nicer handling of default `ParachainSystem` name

* better docs

* Default MockXcm for people who only who don't care to mock xcm.

* cargo fmt

* trailing commas

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* use the variable for hrmp to

* fix deref

* deduplicate MessageQueueChain

* better docs for MessageQueueChain

* Use `Vec<u8>` instead of `&'static [u8]`

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* cargo fmt

* associated changes for using Vec<u8>

* Unused import

* Fix compilation

Co-authored-by: Joshy Orndorff <admin@joshyorndorff.com>
Co-authored-by: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>