core: add a reverse method to Ordering.

This flips the comparison and is designed to be used when sorting etc.
Currently there are no instances of `flip` or `invert` in the standard library, `reverse` is used for mutably reversing a vector, and `rev` is used for reversing a forward iterator. (not much precedent to draw from). So long as it's `#[experimental]` it doesn't matter a whole lot.