pallet-asset-conversion: Swap Credit (#1677)

Introduces a swap implementation that allows the exchange of a credit
(aka Negative Imbalance) of one asset for a credit of another asset.

This is particularly useful when a credit swap is required but may not
have sufficient value to meet the ED constraint, hence cannot be
deposited to temp account before. An example use case is when XCM fees
are paid using an asset held in the XCM executor registry and has to be
swapped for native currency.

Additional Updates:
- encapsulates the existing `Swap` trait impl within a transactional
context, since partial storage mutation is possible when an error
occurs;
- supplied `Currency` and `Assets` impls must be implemented over the
same `Balance` type, the `AssetBalance` generic type is dropped. This
helps to avoid numerous type conversion and overflow cases. If those
types are different it should be handled outside of the pallet;
- `Box` asset kind on a pallet level, unbox on a runtime level - here
[why](https://substrate.stackexchange.com/questions/10039/boxed-argument-of-a-dispatchable/10103#10103);
- `path` uses `Vec` now, instead of `BoundedVec` since it is never used
in PoV;
- removes the `Transfer` event due to it's redundancy with the events
emitted by `fungible/s` implementations;
- modifies the `SwapExecuted` event type;

related issue: 
- https://github.com/paritytech/polkadot-sdk/issues/105

related PRs:
- (required for) https://github.com/paritytech/polkadot-sdk/pull/1845
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1717

// DONE make the pallet work only with `fungibles` trait and make it
free from the concept of a `native` asset -
https://github.com/paritytech/polkadot-sdk/issues/1842

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Co-authored-by: joe petrowski <[email protected]>