Add savestate support

[malleoz]

My current thought is a ISerializable interface where classes must define a serialize and deserialize function. Classes like KartSub should call serialize for the members that have their own serialize impl as well.

Only thing I'm not sure on is when we have to, during runtime, determine whether to load a KartMove object or a KartMoveBike object, for example. Or various different object types. We would maybe need to have an enum to represent an object type so that we know which one to load during runtime.