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/snowbridge/parachain/primitives/core/src/lib.rs

@@ -28,11 +28,7 @@ use sp_core::H256;
 use sp_io::hashing::keccak_256;
 use sp_runtime::{traits::AccountIdConversion, RuntimeDebug};
 use sp_std::prelude::*;
-use xcm::prelude::{
-	Junction::Parachain,
-	Junctions::{Here, X1},
-	MultiLocation,
-};
+use xcm::prelude::{Junction::Parachain, Location};
 use xcm_builder::{DescribeAllTerminal, DescribeFamily, DescribeLocation, HashedDescription};
 
 /// The ID of an agent contract
@@ -53,9 +49,9 @@ pub fn sibling_sovereign_account_raw(para_id: ParaId) -> [u8; 32] {
 }
 
 pub struct AllowSiblingsOnly;
-impl Contains<MultiLocation> for AllowSiblingsOnly {
-	fn contains(location: &MultiLocation) -> bool {
-		matches!(location, MultiLocation { parents: 1, interior: X1(Parachain(_)) })
+impl Contains<Location> for AllowSiblingsOnly {
+	fn contains(location: &Location) -> bool {
+		matches!(location.unpack(), (1, [Parachain(_)]))
 	}
 }
 
@@ -161,14 +157,14 @@ pub const SECONDARY_GOVERNANCE_CHANNEL: ChannelId =
 
 pub struct DescribeHere;
 impl DescribeLocation for DescribeHere {
-	fn describe_location(l: &MultiLocation) -> Option<Vec<u8>> {
-		match (l.parents, l.interior) {
-			(0, Here) => Some(Vec::<u8>::new().encode()),
+	fn describe_location(l: &Location) -> Option<Vec<u8>> {
+		match l.unpack() {
+			(0, []) => Some(Vec::<u8>::new().encode()),
 			_ => None,
 		}
 	}
 }
 
-/// Creates an AgentId from a MultiLocation. An AgentId is a unique mapping to a Agent contract on
-/// Ethereum which acts as the sovereign account for the MultiLocation.
+/// Creates an AgentId from a Location. An AgentId is a unique mapping to a Agent contract on
+/// Ethereum which acts as the sovereign account for the Location.
 pub type AgentIdOf = HashedDescription<H256, (DescribeHere, DescribeFamily<DescribeAllTerminal>)>;