Implement SirenExchange contract, allowing users to purchase options with any ERC20 token

[zareth-san]

See #60 for the design of the contract interface.

In addition to the contract we will need

Solidity tests for SirenExchange
add a deploy script and verify it on Polygon

[zareth-san]

In addition to the design up above, we should add Ethereum-specific functions for buying/selling with ETH. The logic will be almost identical, except the functions should be marked as payable, and instead of sending or receiving an ERC20 token, they will send or receive ETH.

You can use the Solidity code:

someAddress.transfer(amountInWei)

to transfer ETH from the SirenExchange to the msg.sender