ffi: decode output as hex if it looks like a hexstring

[karmacoma-eth]

This is for compatibility with foundry, and will work better with tools like huffc that output hexstrings without a 0x prefix. For instance:

function deployHuff(string memory srcFile) internal returns (address deployedAddress) {
    string[] memory command = new string[](3);
    command[0] = "huffc";
    command[1] = "--bytecode";
    command[2] = srcFile;

    bytes memory bytecode = vm.ffi(command);

    assembly {
        deployedAddress := create(0, add(bytecode, 0x20), mload(bytecode))
    }
}

Before this commit, bytecode was the hexstring. After this commit, bytecode is the actual bytes.