Refund handler implementations

[adambor]

Create refund handlers, covering the basic cases for atomiq.

Timelock refund handler

Allows escrow to be refunded after a specific time in the future.

Example timelock in Solidity

contract Timelock {
    function refund(bytes32 refundData, bytes calldata _witness) view external {
        require(block.timestamp>uint256(refundData), "Not refundable yet");
    }
}

Blockheight refund handler

Allows escrow to be refunded after bitcoin blockchain reaches a specific blockheight. Depends on #2

Example blockheight lock in Solidity

contract BlockheightLock {
    BtcRelay public immutable btcRelay;

    function refund(bytes32 refundData, bytes calldata _witness) view external {
        require(btcRelay.blockheight>=uint256(refundData), "Not refundable yet");
    }
}