Add generic less function for SortSlices

It's a bit of a pain writing a comparison function when we want slices to compare equal regardless of order. How about making SortSlices use a generic less function when it's argument is nil, so that in the common case we can just use SortSlices(nil) to check slice contents regardless of order and type?

The generic less function might be something like this, perhaps: https://play.golang.org/p/NQ1u36jKsi-