rust private

Only call the closure parameter of Iterator::is_sorted_by_key once per item See https://github.com/rust-lang/rust/issues/53485#issuecomment-472314004. This changes `Iterator::is_sorted_by_key` to only call the given closure once for each item, which allows us to pass the items to the closure by value instead of by reference. **Important**: `is_sorted_by_key` for slices and slice iterators is now no longer implemented in terms of the custom `slice::Iter::is_sorted_by` implementation. It's a trade-off: we could forward `slice::Iter::is_sorted_by_key` to it directly for potential SIMD benefits, but that would mean that the closure is potentially called twice for (almost) every element of the slice.