Support asset transfers via XCM

[vgeddes]

Adds support for the following features:

• Upward/Downward transfer of DOT.
• Transferring bridged Ethereum assets between sibling parachains.

Both kinds of transfers are performed using the reserve-backed model as described in Polkadot Cross-Consensus Message (XCM) Format.

Changes:

• Added a pallet token-dealer. This module includes dispatchables for initiating transfers:
  • transfer_dot_to_relaychain
  • transfer_bridged_asset_to_parachain
• Added a crate xcm-support which contains an implementation for XCM TransactAsset trait. This is key to enabling deposits and withdrawals of assets by the XCM executor.
• Iterative improvements to the bridged assets pallet.
• Wiring all the above into the runtime,

There is pretty good unit test coverage for all of this in the the token-dealer module:

vincent@crake:~/code/polkadot-ethereum/parachain/pallets/token-dealer (rococo-v1)$ cargo test
   Compiling artemis-xcm-support v0.1.0 (/home/vincent/code/polkadot-ethereum/parachain/primitives/xcm-support)
   Compiling artemis-token-dealer v0.1.1 (/home/vincent/code/polkadot-ethereum/parachain/pallets/token-dealer)
    Finished test [unoptimized + debuginfo] target(s) in 8.58s
     Running /home/vincent/code/polkadot-ethereum/parachain/target/debug/deps/artemis_token_dealer-10cd52ae3eab93d4

running 3 tests
test tests::transfer_foreign_eth_to_parachain ... ok
test tests::transfer_dot_to_relay_chain ... ok
test tests::transfer_native_eth_to_parachain ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

   Doc-tests artemis-token-dealer

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[vgeddes]

Note: Leaving this PR in draft mode until upstream dependencies have stabilized (i.e. the features we need have been merged into the master branches of Substrate, Polkadot, and Cumulus).

[philipstanislaus]

LGTM, just a few comments 🎉

[philipstanislaus]

Is this change intentional?

[vgeddes]

Yeah, These changes to the files under parachain/node/src/ were necessary to support the CLI interfaces/abstractions provided by the Polkadot in the gav-xcmp branch.

[musnit]

@vgeddes can close?

[vgeddes]

Obsoleted by #200