Serialize the Class enum as a string instead of an int

[2l47]

I feel like this is more legible and usable. Using Serialize instead of Serialize_repr in analyser.rs allows for serde_json::to_string to output instances of the Class enum as a class string instead of an int.

The crate's parse_demo binary still outputs classes as ints due to the usage of ClassList in the default analyser, so the json comparison test cases still pass.

[icewind1991]

Thanks