Flash swaps

[gregdhill]

Is your feature request related to a problem? Please describe.
Liquidators and arbitrageurs need to hold a considerable amount of money in order to their trades. This greatly reduces their relative profits because its very capital inefficient.

Describe the solution you'd like
We need to enable atomic swap transactions (optimistic transactions) to allow traders making liquidations and arbitrage without any capital requirements.

Describe alternatives you've considered
Flash loans would work very similar but would require a second layer (flow: lending protocol -> DEX -> lending protocol) and the maximum swapable amount would reduce to min(capacity[lending], capacity[DEX]).

Additional context
Here is an example of a flash swap implementation on uniswap V2 https://github.com/Uniswap/v2-periphery/blob/master/contracts/examples/ExampleFlashSwap.sol