Deriving cmp::Ord

[Dretch]

It would be useful to be able to derive a cmp::Ord impl, similarly to how you can derive cmp::Eq impls.

[pcwalton]

This is blocked on reforming Ord to use a Less, Equal, Greater enum like Haskell does.

I started work on deriving TotalOrd and have a preliminary version that supports #[deriving_total_ord] for generating an impl based on all of a struct's fields in the order they're defined as well as [#deriving_total_ord(field1, field2, ...)] for restricting/re-ordering the fields used to perform the comparison.

I'll attempt to get a pull request ready in the next few days.

[huonw]

As per #5588, I'm making good progress on this.