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/mocks/msg_queue.rs

@@ -96,16 +96,23 @@ pub mod pallet {
 			max_weight: Weight,
 		) -> Result<Weight, XcmError> {
 			let hash = Encode::using_encoded(&xcm, T::Hashing::hash);
-			let message_hash = Encode::using_encoded(&xcm, sp_io::hashing::blake2_256);
+			let mut message_hash = Encode::using_encoded(&xcm, sp_io::hashing::blake2_256);
 			let (result, event) = match Xcm::<T::RuntimeCall>::try_from(xcm) {
 				Ok(xcm) => {
 					let location = (Parent, Parachain(sender.into()));
-					match T::XcmExecutor::execute_xcm(location, xcm, message_hash, max_weight) {
-						Outcome::Error(e) => (Err(e), Event::Fail(Some(hash), e)),
-						Outcome::Complete(w) => (Ok(w), Event::Success(Some(hash))),
+					match T::XcmExecutor::prepare_and_execute(
+						location,
+						xcm,
+						&mut message_hash,
+						max_weight,
+						Weight::zero(),
+					) {
+						Outcome::Error { error } => (Err(error), Event::Fail(Some(hash), error)),
+						Outcome::Complete { used } => (Ok(used), Event::Success(Some(hash))),
 						// As far as the caller is concerned, this was dispatched without error, so
 						// we just report the weight used.
-						Outcome::Incomplete(w, e) => (Ok(w), Event::Fail(Some(hash), e)),
+						Outcome::Incomplete { used, error } =>
+							(Ok(used), Event::Fail(Some(hash), error)),
 					}
 				},
 				Err(()) => (Err(XcmError::UnhandledXcmVersion), Event::BadVersion(Some(hash))),
@@ -146,7 +153,7 @@ pub mod pallet {
 			limit: Weight,
 		) -> Weight {
 			for (_i, (_sent_at, data)) in iter.enumerate() {
-				let id = sp_io::hashing::blake2_256(&data[..]);
+				let mut id = sp_io::hashing::blake2_256(&data[..]);
 				let maybe_versioned = VersionedXcm::<T::RuntimeCall>::decode(&mut &data[..]);
 				match maybe_versioned {
 					Err(_) => {
@@ -155,7 +162,13 @@ pub mod pallet {
 					Ok(versioned) => match Xcm::try_from(versioned) {
 						Err(()) => Self::deposit_event(Event::UnsupportedVersion(id)),
 						Ok(x) => {
-							let outcome = T::XcmExecutor::execute_xcm(Parent, x.clone(), id, limit);
+							let outcome = T::XcmExecutor::prepare_and_execute(
+								Parent,
+								x.clone(),
+								&mut id,
+								limit,
+								Weight::zero(),
+							);
 							<ReceivedDmp<T>>::append(x);
 							Self::deposit_event(Event::ExecutedDownward(id, outcome));
 						},