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>

substrate/frame/contracts/mock-network/src/lib.rs

@@ -24,7 +24,7 @@ mod tests;
 
 use crate::primitives::{AccountId, UNITS};
 use sp_runtime::BuildStorage;
-use xcm::latest::{prelude::*, MultiLocation};
+use xcm::latest::prelude::*;
 use xcm_executor::traits::ConvertLocation;
 use xcm_simulator::{decl_test_network, decl_test_parachain, decl_test_relay_chain, TestExt};
 
@@ -67,12 +67,12 @@ decl_test_network! {
 }
 
 pub fn relay_sovereign_account_id() -> AccountId {
-	let location: MultiLocation = (Parent,).into();
+	let location: Location = (Parent,).into();
 	parachain::SovereignAccountOf::convert_location(&location).unwrap()
 }
 
 pub fn parachain_sovereign_account_id(para: u32) -> AccountId {
-	let location: MultiLocation = (Parachain(para),).into();
+	let location: Location = (Parachain(para),).into();
 	relay_chain::SovereignAccountOf::convert_location(&location).unwrap()
 }
 
@@ -80,7 +80,7 @@ pub fn parachain_account_sovereign_account_id(
 	para: u32,
 	who: sp_runtime::AccountId32,
 ) -> AccountId {
-	let location: MultiLocation = (
+	let location: Location = (
 		Parachain(para),
 		AccountId32 { network: Some(relay_chain::RelayNetwork::get()), id: who.into() },
 	)