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/primitives/utility/src/tests/swap_first.rs

@@ -18,18 +18,18 @@ use crate::*;
 use frame_support::{parameter_types, traits::fungibles::Inspect};
 use mock::{setup_pool, AccountId, AssetId, Balance, Fungibles};
 use xcm::latest::AssetId as XcmAssetId;
-use xcm_executor::Assets as HoldingAsset;
+use xcm_executor::AssetsInHolding;
 
-fn create_holding_asset(asset_id: AssetId, amount: Balance) -> HoldingAsset {
+fn create_holding_asset(asset_id: AssetId, amount: Balance) -> AssetsInHolding {
 	create_asset(asset_id, amount).into()
 }
 
-fn create_asset(asset_id: AssetId, amount: Balance) -> MultiAsset {
-	MultiAsset { id: create_asset_id(asset_id), fun: Fungible(amount) }
+fn create_asset(asset_id: AssetId, amount: Balance) -> Asset {
+	Asset { id: create_asset_id(asset_id), fun: Fungible(amount) }
 }
 
 fn create_asset_id(asset_id: AssetId) -> XcmAssetId {
-	Concrete(MultiLocation::new(0, X1(GeneralIndex(asset_id.into()))))
+	AssetId(Location::new(0, [GeneralIndex(asset_id.into())]))
 }
 
 fn xcm_context() -> XcmContext {
@@ -319,7 +319,7 @@ fn empty_holding_asset() {
 	let mut trader = Trader::new();
 	assert_eq!(
 		trader
-			.buy_weight(Weight::from_all(10), HoldingAsset::new(), &xcm_context())
+			.buy_weight(Weight::from_all(10), AssetsInHolding::new(), &xcm_context())
 			.unwrap_err(),
 		XcmError::AssetNotFound
 	);
@@ -328,8 +328,7 @@ fn empty_holding_asset() {
 #[test]
 fn fails_to_match_holding_asset() {
 	let mut trader = Trader::new();
-	let holding_asset =
-		MultiAsset { id: Concrete(MultiLocation::new(1, X1(Parachain(1)))), fun: Fungible(10) };
+	let holding_asset = Asset { id: AssetId(Location::new(1, [Parachain(1)])), fun: Fungible(10) };
 	assert_eq!(
 		trader
 			.buy_weight(Weight::from_all(10), holding_asset.into(), &xcm_context())
@@ -536,14 +535,15 @@ pub mod mock {
 	pub struct FungiblesMatcher;
 	impl MatchesFungibles<AssetId, Balance> for FungiblesMatcher {
 		fn matches_fungibles(
-			a: &MultiAsset,
+			a: &Asset,
 		) -> core::result::Result<(AssetId, Balance), xcm_executor::traits::Error> {
 			match a {
-				MultiAsset {
-					fun: Fungible(amount),
-					id:
-						Concrete(MultiLocation { parents: 0, interior: X1(Junction::GeneralIndex(id)) }),
-				} => Ok(((*id).try_into().unwrap(), *amount)),
+				Asset { fun: Fungible(amount), id: AssetId(inner_location) } =>
+					match inner_location.unpack() {
+						(0, [Junction::GeneralIndex(id)]) =>
+							Ok(((*id).try_into().unwrap(), *amount)),
+						_ => Err(xcm_executor::traits::Error::AssetNotHandled),
+					},
 				_ => Err(xcm_executor::traits::Error::AssetNotHandled),
 			}
 		}