NSSecureCoding instead of NSCoding

[baxissimo]

Would it make sense for this project to use NSSecureCoding instead of NSCoding?
I'm not sure what all the issues are surrounding this, I just know our code base has a test that tries to make sure all the classes we depend on DO NOT use NSCoding, and it is finding the NSCoding usages in this project.

[wuhao5]

Yes. There wasn't an issue to not use NSSecureCoding but only for a quick serialization solution. If you have better solutions, any PR is welcomed!