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/xcm/xcm-builder/src/origin_aliases.rs

@@ -14,7 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
-//! Implementation for `ContainsPair<MultiLocation, MultiLocation>`.
+//! Implementation for `ContainsPair<Location, Location>`.
 
 use frame_support::traits::{Contains, ContainsPair};
 use sp_std::marker::PhantomData;
@@ -25,13 +25,15 @@ use xcm::latest::prelude::*;
 ///
 /// Requires that the prefixed origin `AccountId32` matches the target `AccountId32`.
 pub struct AliasForeignAccountId32<Prefix>(PhantomData<Prefix>);
-impl<Prefix: Contains<MultiLocation>> ContainsPair<MultiLocation, MultiLocation>
+impl<Prefix: Contains<Location>> ContainsPair<Location, Location>
 	for AliasForeignAccountId32<Prefix>
 {
-	fn contains(origin: &MultiLocation, target: &MultiLocation) -> bool {
-		if let (prefix, Some(account_id @ AccountId32 { .. })) = origin.split_last_interior() {
+	fn contains(origin: &Location, target: &Location) -> bool {
+		if let (prefix, Some(account_id @ AccountId32 { .. })) =
+			origin.clone().split_last_interior()
+		{
 			return Prefix::contains(&prefix) &&
-				*target == MultiLocation { parents: 0, interior: X1(account_id) }
+				*target == Location { parents: 0, interior: [account_id].into() }
 		}
 		false
 	}