Sorted predicates

As discussed in #6, there is room for a `Sorted` predicate. I'm currently running into use cases where an algorithm's implementation differs for sorted and unsorted collections. If we can decide on a type signature, I can start with the implementation. With regards to the sort direction, in my opinion we can do two things:

1. we can just assume ascending.
2. we can provide some sort of enum type which describes the direction.

I'd like to hear opinions on this. Also, our implicit `Validate` will need some sort of (implicit) `Ordering` to decide if the collection is sorted or not.