At first I was planning on making `CompareTo` implemented at
`IRulesetInfo` itself and shared across classes, but turns out it only
implements it explicitly and not allow direct `IRulesetInfo.Equals`
calls.
It messed with my head enough that I decided to just let each class have
its own implementation and only allow same type.
They're mostly used in extensibility scenarios, so everything happens in
runtime. There is no better resolution than to crash with a null
reference exception.