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/runtime/test-runtime/src/lib.rs

@@ -604,7 +604,7 @@ pub mod pallet_test_notifier {
 	pub enum Event<T: Config> {
 		QueryPrepared(QueryId),
 		NotifyQueryPrepared(QueryId),
-		ResponseReceived(MultiLocation, QueryId, Response),
+		ResponseReceived(Location, QueryId, Response),
 	}
 
 	#[pallet::error]

polkadot/runtime/test-runtime/src/xcm_config.rs

@@ -27,7 +27,7 @@ use xcm_builder::{
 };
 use xcm_executor::{
 	traits::{TransactAsset, WeightTrader},
-	Assets,
+	AssetsInHolding,
 };
 
 parameter_types! {
@@ -35,10 +35,10 @@ parameter_types! {
 	pub const AnyNetwork: Option<NetworkId> = None;
 	pub const MaxInstructions: u32 = 100;
 	pub const MaxAssetsIntoHolding: u32 = 16;
-	pub const UniversalLocation: xcm::latest::InteriorMultiLocation = xcm::latest::Junctions::Here;
+	pub const UniversalLocation: xcm::latest::InteriorLocation = xcm::latest::Junctions::Here;
 }
 
-/// Type to convert an `Origin` type value into a `MultiLocation` value which represents an interior
+/// Type to convert an `Origin` type value into a `Location` value which represents an interior
 /// location of this chain.
 pub type LocalOriginToLocation = (
 	// And a usual Signed origin to be used in XCM as a corresponding AccountId32
@@ -48,8 +48,8 @@ pub type LocalOriginToLocation = (
 pub struct DoNothingRouter;
 impl SendXcm for DoNothingRouter {
 	type Ticket = ();
-	fn validate(_dest: &mut Option<MultiLocation>, _msg: &mut Option<Xcm<()>>) -> SendResult<()> {
-		Ok(((), MultiAssets::new()))
+	fn validate(_dest: &mut Option<Location>, _msg: &mut Option<Xcm<()>>) -> SendResult<()> {
+		Ok(((), Assets::new()))
 	}
 	fn deliver(_: ()) -> Result<XcmHash, SendError> {
 		Ok([0; 32])
@@ -60,20 +60,16 @@ pub type Barrier = AllowUnpaidExecutionFrom<Everything>;
 
 pub struct DummyAssetTransactor;
 impl TransactAsset for DummyAssetTransactor {
-	fn deposit_asset(
-		_what: &MultiAsset,
-		_who: &MultiLocation,
-		_context: Option<&XcmContext>,
-	) -> XcmResult {
+	fn deposit_asset(_what: &Asset, _who: &Location, _context: Option<&XcmContext>) -> XcmResult {
 		Ok(())
 	}
 
 	fn withdraw_asset(
-		_what: &MultiAsset,
-		_who: &MultiLocation,
+		_what: &Asset,
+		_who: &Location,
 		_maybe_context: Option<&XcmContext>,
-	) -> Result<Assets, XcmError> {
-		let asset: MultiAsset = (Parent, 100_000).into();
+	) -> Result<AssetsInHolding, XcmError> {
+		let asset: Asset = (Parent, 100_000).into();
 		Ok(asset.into())
 	}
 }
@@ -87,10 +83,10 @@ impl WeightTrader for DummyWeightTrader {
 	fn buy_weight(
 		&mut self,
 		_weight: Weight,
-		_payment: Assets,
+		_payment: AssetsInHolding,
 		_context: &XcmContext,
-	) -> Result<Assets, XcmError> {
-		Ok(Assets::default())
+	) -> Result<AssetsInHolding, XcmError> {
+		Ok(AssetsInHolding::default())
 	}
 }