I know we can't have `finally` as the method name as it is reserved, but `finally0` seems awkward. Other than Action0 (because it has different arities) what is the reasoning behind using 0 at the end? I don't think `finally` will ever have different arities, and if it did it would just have overloads with more arguments.

We likely want to keep the prefix `finally` since that's what people will be looking for coming from other Rx implementations so perhaps something like `finallyDo`?