XCMv4 (#1230)

# Note for reviewer

Most changes are just syntax changes necessary for the new version.
Most important files should be the ones under the `xcm` folder.

# Description 

Added XCMv4.

## Removed `Multi` prefix
The following types have been renamed:
- MultiLocation -> Location
- MultiAsset -> Asset
- MultiAssets -> Assets
- InteriorMultiLocation -> InteriorLocation
- MultiAssetFilter -> AssetFilter
- VersionedMultiAsset -> VersionedAsset
- WildMultiAsset -> WildAsset
- VersionedMultiLocation -> VersionedLocation

In order to fix a name conflict, the `Assets` in `xcm-executor` were
renamed to `HoldingAssets`, as they represent assets in holding.

## Removed `Abstract` asset id

It was not being used anywhere and this simplifies the code.

Now assets are just constructed as follows:

```rust
let asset: Asset = (AssetId(Location::new(1, Here)), 100u128).into();
```

No need for specifying `Concrete` anymore.

## Outcome is now a named fields struct

Instead of

```rust
pub enum Outcome {
  Complete(Weight),
  Incomplete(Weight, Error),
  Error(Error),
}
```

we now have

```rust
pub enum Outcome {
  Complete { used: Weight },
  Incomplete { used: Weight, error: Error },
  Error { error: Error },
}
```

## Added Reanchorable trait

Now both locations and assets implement this trait, making it easier to
reanchor both.

## New syntax for building locations and junctions

Now junctions are built using the following methods:

```rust
let location = Location {
    parents: 1,
    interior: [Parachain(1000), PalletInstance(50), GeneralIndex(1984)].into()
};
```

or

```rust
let location = Location::new(1, [Parachain(1000), PalletInstance(50), GeneralIndex(1984)]);
```

And they are matched like so:

```rust
match location.unpack() {
  (1, [Parachain(id)]) => ...
  (0, Here) => ...,
  (1, [_]) => ...,
}
```

This syntax is mandatory in v4, and has been also implemented for v2 and
v3 for easier migration.

This was needed to make all sizes smaller.

# TODO
- [x] Scaffold v4
- [x] Port github.com/paritytech/polkadot/pull/7236
- [x] Remove `Multi` prefix
- [x] Remove `Abstract` asset id

---------

Co-authored-by: command-bot <>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

cumulus/parachains/runtimes/bridge-hubs/common/src/message_queue.rs

@@ -23,7 +23,7 @@ use pallet_message_queue::OnQueueChanged;
 use scale_info::TypeInfo;
 use snowbridge_core::ChannelId;
 use sp_std::{marker::PhantomData, prelude::*};
-use xcm::v3::{Junction, MultiLocation};
+use xcm::v4::{Junction, Location};
 
 /// The aggregate origin of an inbound message.
 /// This is specialized for BridgeHub, as the snowbridge-outbound-queue-pallet is also using
@@ -46,16 +46,16 @@ pub enum AggregateMessageOrigin {
 	Snowbridge(ChannelId),
 }
 
-impl From<AggregateMessageOrigin> for MultiLocation {
+impl From<AggregateMessageOrigin> for Location {
 	fn from(origin: AggregateMessageOrigin) -> Self {
 		use AggregateMessageOrigin::*;
 		match origin {
-			Here => MultiLocation::here(),
-			Parent => MultiLocation::parent(),
-			Sibling(id) => MultiLocation::new(1, Junction::Parachain(id.into())),
+			Here => Location::here(),
+			Parent => Location::parent(),
+			Sibling(id) => Location::new(1, Junction::Parachain(id.into())),
 			// NOTE: We don't need this conversion for Snowbridge. However we have to
 			// implement it anyway as xcm_builder::ProcessXcmMessage requires it.
-			Snowbridge(_) => MultiLocation::default(),
+			Snowbridge(_) => Location::default(),
 		}
 	}
 }