Flash swap feature

[Alexangelj]

Flash swaps will optimistically send tokens out during swaps before the input tokens are requested. This will enable the output of a swap to be used for payments in other integrated protocols.

Changes:

• Adds callbackData argument to swap(). This argument is passed to the callback() to msg.sender. If the length of data is 0, it will not do the callback and instead use _transferFrom().
• Adds the ISwapCallback interface to define the callback function.
• Re-orders the business logic of the swap token transfers to transfer the tokens to recipient before the input tokens are required/requested.

Concerns:

• cross pool interaction using flash swaps
• incorrect usage of callback
• arbitrary data passed through the swap entrypoint is arbitrary (but only used in the callback call)

[Alexangelj]

Do not merge until the compilation is fixed. fighting stack too deep

[clemlak]

I fixed the stack too deep error, a small issue related to WAD units and added a couple of tests. We should be good now.