Add Xcm sender.

[shaunxw]

Fixes #2405

[shawntabrizi]

This doesn't look like the right place for this.

You should implement such a thing here: https://github.com/paritytech/polkadot/tree/master/xcm

With a more specific struct name saying how it should be used, and generic usage of the xcm executor.

[shaunxw]

This doesn't look like the right place for this.

You should implement such a thing here: https://github.com/paritytech/polkadot/tree/master/xcm

I thought this place is only for fundamental XCM definitions that could be used across relay chain and parachain impl.

The XCM sender defined in this PR is only for relay chain's use, as it only do DMP. That's why I put it in common runtime lib. IMHO the relay chain XCM sender might be in a place where less chance developers would use a wrong sender, and I guess this is also the reason the XCM sender which do UMP and HRMP is defined in cumulus, instead of here.

With a more specific struct name saying how it should be used, and generic usage of the xcm executor.

Will update the naming and documentation.

[shaunxw]

Another thing I forgot to mention is that this sender depends on dmp and configuration module. It's not possible make it in https://github.com/paritytech/polkadot/tree/master/xcm unless defining traits to abstract dmp and configuration behaviors.

[shawntabrizi]

I think this is fine for now then, but long term, I suspect this will be refactored somewhere else.

I don't have a great immediate suggestion.

[shaunxw]

Looks like this PR needs more approvals as @bkchr added A0-pleasereview label.

Please help to add reviewers. Thanks!

[shaunxw]

@shawntabrizi @bkchr Could you please help to add reviewers?

[shawntabrizi]

bot merge

[ghost]

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