Fix handling of empty range

[qsantos]

Currently, comparing a value to the empty set yields different results, depending on how it is represented:

        assert_eq!(30.range_cmp(45..35), RangeOrdering::Below);
        assert_eq!(30.range_cmp(25..15), RangeOrdering::Above);

If we want to follow the semantics of the set of values represented by the range, none of these is actually correct, and we should add a new variant for RangeOrdering for this case.

We could also decide that we want to keep this behavior, since the representations 45..35 and 25..15 actually represent different objects.

[qsantos]

Note that ..0u32 is empty. Have fun implementing is_empty over arbitrary RangeBounds.