XCM: Automatic Version Negotiation (#3736)

* XCM: Automatic Version Negotiation

* Introduce the version instructions and subscription trait

* Notification and subscription data migration

* Version change subscriptions

* Fixes

* Formatting

* Spelling

* Fixes

* Fixes

* Automatic unsubscription

* Formatting

* Expose remote origin in VM and ensure it is unchanged from actual origin in subscription instructions.

* Barrier

* Unsubscription extrinsic

* Remove top_level param

* Formatting

* Fixes

* Automatic subscription

* Formatting

* Spelling

* Unit tests for XCM executor

* Formatting

* Spellin

* Unit test for XCM pallet subscriber side

* Formatting

* More tests

* Formatting

* Fixes

* Subscription-side tests

* Formatting

* Unit tests for XCM pallet

* Formatting

* Update roadmap/implementers-guide/src/types/overseer-protocol.md

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

* Remove commented code

* Grumbles

* Multi-stage XCM version migration

* Formatting

* v1 subscriptions backport

* Warning

* Spelling

* Fix grumbles

* Formatting

* Avoid running through old notifications

* Formatting

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

polkadot/roadmap/implementers-guide/src/types/overseer-protocol.md

@@ -441,7 +441,7 @@ enum DisputeCoordinatorMessage {
         pending_confirmation: oneshot::Sender<ImportStatementsResult>
     },
     /// Fetch a list of all recent disputes that the co-ordinator is aware of.
-    /// These are disputes which have occured any time in recent sessions, which may have already concluded.
+    /// These are disputes which have occurred any time in recent sessions, which may have already concluded.
     RecentDisputes(ResponseChannel<Vec<(SessionIndex, CandidateHash)>>),
     /// Fetch a list of all active disputes that the co-ordinator is aware of.
     /// These disputes are either unconcluded or recently concluded.
@@ -699,7 +699,7 @@ The Runtime API subsystem is responsible for providing an interface to the state
 
 This is fueled by an auxiliary type encapsulating all request types defined in the Runtime API section of the guide.
 
-> TODO: link to the Runtime API section. Not possible currently because of https://github.com/Michael-F-Bryan/mdbook-linkcheck/issues/25. Once v0.7.1 is released it will work.
+> To do: link to the Runtime API section. Not possible currently because of https://github.com/Michael-F-Bryan/mdbook-linkcheck/issues/25. Once v0.7.1 is released it will work.
 
 ```rust
 enum RuntimeApiRequest {