Guide: Derive Eq to avoid compile error.

If we copy in `Ordering` and use it with the following example we'll get a compile error as `==` has not been defined.

I chose to introduce `deriving(Eq, PartialEq)` but refer to the later discussion (which I assume is going to come). I'm not sure about the formulation however.

It could have been fine to use `match`, but it hasn't been presented yet. In the next example though we *are* using `match` in the example code, so we could do it here as well. But I'm more inclined to remove the use of `match` there as well as it might be a bit confusing?

cc @steveklabnik