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>

polkadot/xcm/xcm-builder/src/pay.rs

@@ -30,7 +30,7 @@ use xcm_executor::traits::{QueryHandler, QueryResponseStatus};
 /// ownership of some `Interior` location of the local chain to a particular `Beneficiary`. The
 /// `AssetKind` value is not itself bounded (to avoid the issue of needing to wrap some preexisting
 /// datatype), however a converter type `AssetKindToLocatableAsset` must be provided in order to
-/// translate it into a `LocatableAsset`, which comprises both an XCM `MultiLocation` describing
+/// translate it into a `LocatableAsset`, which comprises both an XCM `Location` describing
 /// the XCM endpoint on which the asset to be paid resides and an XCM `AssetId` to identify the
 /// specific asset at that endpoint.
 ///
@@ -65,14 +65,14 @@ pub struct PayOverXcm<
 	)>,
 );
 impl<
-		Interior: Get<InteriorMultiLocation>,
+		Interior: Get<InteriorLocation>,
 		Router: SendXcm,
 		Querier: QueryHandler,
 		Timeout: Get<Querier::BlockNumber>,
 		Beneficiary: Clone,
 		AssetKind,
 		AssetKindToLocatableAsset: TryConvert<AssetKind, LocatableAssetId>,
-		BeneficiaryRefToLocation: for<'a> TryConvert<&'a Beneficiary, MultiLocation>,
+		BeneficiaryRefToLocation: for<'a> TryConvert<&'a Beneficiary, Location>,
 	> Pay
 	for PayOverXcm<
 		Interior,
@@ -105,7 +105,7 @@ impl<
 		let beneficiary = BeneficiaryRefToLocation::try_convert(&who)
 			.map_err(|_| xcm::latest::Error::InvalidLocation)?;
 
-		let query_id = Querier::new_query(asset_location, Timeout::get(), Interior::get());
+		let query_id = Querier::new_query(asset_location.clone(), Timeout::get(), Interior::get());
 
 		let message = Xcm(vec![
 			DescendOrigin(Interior::get()),
@@ -120,8 +120,7 @@ impl<
 			])),
 			TransferAsset {
 				beneficiary,
-				assets: vec![MultiAsset { id: asset_id, fun: Fungibility::Fungible(amount) }]
-					.into(),
+				assets: vec![Asset { id: asset_id, fun: Fungibility::Fungible(amount) }].into(),
 			},
 		]);
 
@@ -195,16 +194,16 @@ pub struct LocatableAssetId {
 	/// The asset's ID.
 	pub asset_id: AssetId,
 	/// The (relative) location in which the asset ID is meaningful.
-	pub location: MultiLocation,
+	pub location: Location,
 }
 
 /// Adapter `struct` which implements a conversion from any `AssetKind` into a [`LocatableAssetId`]
 /// value using a fixed `Location` for the `location` field.
-pub struct FixedLocation<Location>(sp_std::marker::PhantomData<Location>);
-impl<Location: Get<MultiLocation>, AssetKind: Into<AssetId>> TryConvert<AssetKind, LocatableAssetId>
-	for FixedLocation<Location>
+pub struct FixedLocation<FixedLocationValue>(sp_std::marker::PhantomData<FixedLocationValue>);
+impl<FixedLocationValue: Get<Location>, AssetKind: Into<AssetId>>
+	TryConvert<AssetKind, LocatableAssetId> for FixedLocation<FixedLocationValue>
 {
 	fn try_convert(value: AssetKind) -> Result<LocatableAssetId, AssetKind> {
-		Ok(LocatableAssetId { asset_id: value.into(), location: Location::get() })
+		Ok(LocatableAssetId { asset_id: value.into(), location: FixedLocationValue::get() })
 	}
 }