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/collectives/collectives-westend/src/lib.rs

@@ -427,7 +427,7 @@ impl cumulus_pallet_aura_ext::Config for Runtime {}
 
 parameter_types! {
 	/// The asset ID for the asset that we use to pay for message delivery fees.
-	pub FeeAssetId: AssetId = Concrete(xcm_config::WndLocation::get());
+	pub FeeAssetId: AssetId = AssetId(xcm_config::WndLocation::get());
 	/// The base fee for the message delivery fees.
 	pub const BaseDeliveryFee: u128 = CENTS.saturating_mul(3);
 }
@@ -974,28 +974,28 @@ impl_runtime_apis! {
 
 			use pallet_xcm::benchmarking::Pallet as PalletXcmExtrinsicsBenchmark;
 			impl pallet_xcm::benchmarking::Config for Runtime {
-				fn reachable_dest() -> Option<MultiLocation> {
+				fn reachable_dest() -> Option<Location> {
 					Some(Parent.into())
 				}
 
-				fn teleportable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+				fn teleportable_asset_and_dest() -> Option<(Asset, Location)> {
 					// Relay/native token can be teleported between Collectives and Relay.
 					Some((
-						MultiAsset {
+						Asset {
 							fun: Fungible(EXISTENTIAL_DEPOSIT),
-							id: Concrete(Parent.into())
+							id: AssetId(Parent.into())
 						}.into(),
 						Parent.into(),
 					))
 				}
 
-				fn reserve_transferable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+				fn reserve_transferable_asset_and_dest() -> Option<(Asset, Location)> {
 					// Reserve transfers are disabled on Collectives.
 					None
 				}
 
 				fn set_up_complex_asset_transfer(
-				) -> Option<(MultiAssets, u32, MultiLocation, Box<dyn FnOnce()>)> {
+				) -> Option<(Assets, u32, Location, Box<dyn FnOnce()>)> {
 					// Collectives only supports teleports to system parachain.
 					// Relay/native token can be teleported between Collectives and Relay.
 					let native_location = Parent.into();