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>

bridges/bin/runtime-common/src/messages_benchmarking.rs

@@ -38,7 +38,7 @@ use frame_support::weights::Weight;
 use pallet_bridge_messages::benchmarking::{MessageDeliveryProofParams, MessageProofParams};
 use sp_runtime::traits::{Header, Zero};
 use sp_std::prelude::*;
-use xcm::v3::prelude::*;
+use xcm::latest::prelude::*;
 
 /// Prepare inbound bridge message according to given message proof parameters.
 fn prepare_inbound_message(
@@ -266,19 +266,19 @@ where
 /// Returns callback which generates `BridgeMessage` from Polkadot XCM builder based on
 /// `expected_message_size` for benchmark.
 pub fn generate_xcm_builder_bridge_message_sample(
-	destination: InteriorMultiLocation,
+	destination: InteriorLocation,
 ) -> impl Fn(usize) -> MessagePayload {
 	move |expected_message_size| -> MessagePayload {
 		// For XCM bridge hubs, it is the message that
 		// will be pushed further to some XCM queue (XCMP/UMP)
-		let location = xcm::VersionedInteriorMultiLocation::V3(destination);
+		let location = xcm::VersionedInteriorLocation::V4(destination.clone());
 		let location_encoded_size = location.encoded_size();
 
 		// we don't need to be super-precise with `expected_size` here
 		let xcm_size = expected_message_size.saturating_sub(location_encoded_size);
 		let xcm_data_size = xcm_size.saturating_sub(
 			// minus empty instruction size
-			xcm::v3::Instruction::<()>::ExpectPallet {
+			Instruction::<()>::ExpectPallet {
 				index: 0,
 				name: vec![],
 				module_name: vec![],
@@ -294,8 +294,8 @@ pub fn generate_xcm_builder_bridge_message_sample(
 			expected_message_size, location_encoded_size, xcm_size, xcm_data_size,
 		);
 
-		let xcm = xcm::VersionedXcm::<()>::V3(
-			vec![xcm::v3::Instruction::<()>::ExpectPallet {
+		let xcm = xcm::VersionedXcm::<()>::V4(
+			vec![Instruction::<()>::ExpectPallet {
 				index: 0,
 				name: vec![42; xcm_data_size],
 				module_name: vec![],