func distance<T : ForwardIndexType>(_: T, end: T) Measure the distance between start and end. If T models RandomAccessIndexType, requires that start and end are part of the same sequence, and executes in O(1). Otherwise, requires that end is reachable from start by incrementation, and executes in O(N), where N is the function's result. Declaration func distance<T : ForwardIndexType>(start: T, end: T) -> T.Distance