XCM v3: Context & ID hash

[gavofyork]

Simple refactor ensuring that send_xcm returns the hash of the message as sent.

This should be combined with a new context system to allow the functions in traits used by XcmExecutor to have knowledge of the context of their instruction. The context passed into the trait functions should include at least the origin: Option<MultiLocation> and message_hash: XcmHash. E.g.:

pub struct XcmContext {
  origin: Option<MultiLocation>,
  message_hash: XcmHash,
}

Traits designed only for use with XCM can have this passed in to their relevant functions. Traits not designed for use only in XCM can have converter structs which discard the context before forwarding.

Topic

There should be a new item in the XCVM: topic: Option<[u8; 32]>. Context should be extended to include a topic: Option<[u8; 32]> field, representing this value. There should be two new instructions:

- `SetTopic([u8; 32])`: Sets the XCVM's topic to `Some`.
- `ResetTopic`: Sets the XCVM's topic to `None`.

Code Migration

XcmContext

Several APIs now accept an additional context: &XcmContext parameter (except in the case of withdraw_asset, which accepts a maybe_context: Option<&XcmContext>).

This parameter must be added, but its value may be ignored.

XcmHash

SendXcm and ExportXcm must both provide an XcmHash value back from deliver function if it returns Ok. It is up to the trait implementation exactly how this hash is determined, however in general it should identify the message which was sent and should be identifiable on both sides of the transport. Generally this will mean hashing the data which is encoded and set on the wire (or the common part of it, if it changes over the transport).

XcmExecutor

XcmExecutor::execute has now been renamed to XcmExecutor::bench_process and is only enabled for benchmarks. Prior to this it has collided with the trait function ExecuteXcm::execute.

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/substrate-etl-polkadot-xcmtransfers-dataset-xcmv3-context-matching/1732/1