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/assets/common/src/benchmarks.rs

@@ -19,26 +19,25 @@ use sp_std::marker::PhantomData;
 use xcm::latest::prelude::*;
 
 /// Creates asset pairs for liquidity pools with `Target` always being the first asset.
-pub struct AssetPairFactory<Target, SelfParaId, PalletId>(
-	PhantomData<(Target, SelfParaId, PalletId)>,
+pub struct AssetPairFactory<Target, SelfParaId, PalletId, L = Location>(
+	PhantomData<(Target, SelfParaId, PalletId, L)>,
 );
-impl<Target: Get<MultiLocation>, SelfParaId: Get<ParaId>, PalletId: Get<u32>>
-	pallet_asset_conversion::BenchmarkHelper<MultiLocation>
-	for AssetPairFactory<Target, SelfParaId, PalletId>
+impl<Target: Get<L>, SelfParaId: Get<ParaId>, PalletId: Get<u32>, L: TryFrom<Location>>
+	pallet_asset_conversion::BenchmarkHelper<L> for AssetPairFactory<Target, SelfParaId, PalletId, L>
 {
-	fn create_pair(seed1: u32, seed2: u32) -> (MultiLocation, MultiLocation) {
-		let with_id = MultiLocation::new(
+	fn create_pair(seed1: u32, seed2: u32) -> (L, L) {
+		let with_id = Location::new(
 			1,
-			X3(
+			[
 				Parachain(SelfParaId::get().into()),
 				PalletInstance(PalletId::get() as u8),
 				GeneralIndex(seed2.into()),
-			),
+			],
 		);
 		if seed1 % 2 == 0 {
-			(with_id, Target::get())
+			(with_id.try_into().map_err(|_| "Something went wrong").unwrap(), Target::get())
 		} else {
-			(Target::get(), with_id)
+			(Target::get(), with_id.try_into().map_err(|_| "Something went wrong").unwrap())
 		}
 	}
 }