Girazoki dual token transferrable

[girazoki]

This is a draft proposal (still many things to polish) for being able to send distinct asset and fee assets, as long as they have the same reserve chain. This is intended for chains that only accept a small subset of assets as for xcm fee payment. For instance. imagine Parachain A has pallet_assets with an asset AssetA and pallet_balances with its native token is NativeA.

A reciving parachain B would only need to support xcm payments in NativeA, and thus if one wants to send AssetA it would need to send also NativeA for fee payment. This is intended to potentially limit the assets a parachain supports for fee payment.

Additionally, a refactor is included in preparation for the possibility to, in the future, send more than just 2 tokens to a chain

[codecov-commenter]

Codecov Report

Merging #683 (5a59b21) into master (c283793) will decrease coverage by 0.35%.
The diff coverage is 63.95%.

❗ Current head 5a59b21 differs from pull request most recent head 28636a8. Consider uploading reports for the commit 28636a8 to get more accurate results

@@            Coverage Diff             @@
##           master     #683      +/-   ##
==========================================
- Coverage   74.80%   74.44%   -0.36%     
==========================================
  Files          82       82              
  Lines        7073     7150      +77     
==========================================
+ Hits         5291     5323      +32     
- Misses       1782     1827      +45     

Impacted Files	Coverage Δ
xtokens/src/lib.rs	58.07% <48.76%> (-12.76%)	⬇️
xtokens/src/mock/mod.rs	93.47% <100.00%> (+0.97%)	⬆️
xtokens/src/tests.rs	98.97% <100.00%> (+0.12%)	⬆️

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[girazoki]

Several tests to be added yet (specially for errors) but major changes have been:

• A unique function where we calculate the transfer_kind, and also unique instruction functions to which we pass multiassets now.
• Addition of two new extrinsics that allow to transfer up to two distinct assets as long as they share the same reserve
• Removal of the ArithmeticOverflow error and use the MultiAssets push function instead, which already uses saturated addition