Fix Eclipse compiler warnings

I've split the various fixes into separate commits to illustrate why/how I fixed certain warnings.
I think the warning makes sense. If we paid attention to it in the first place, we probably would have never shipped the class without an explicit `serialVersionUID`. Now that we have I don't want to break serialization compatibility by introducing an `serialVersionUID`.

If we generate one, it might be different from the one used by another JVM if I read the [spec](https://docs.oracle.com/javase/7/docs/platform/serialization/spec/class.html#4100) correctly. If we added a custom one (like `1L`), it would be different for sure.