Add support for Airswap's swap protocol and AST

Airswap is a non-custodial DEX that supports a number of ERC20 tokens and requires its own native token to be used for makers (not sure if that would be compatible with the arbitrage mechanism used by the Marble protocol. Here are links to Airswap:

https://www.airswap.io/
https://swap.tech/whitepaper/