This dramatically improves the performance of skip() on arrays by getting
rid of the type instability which is currently not well handled.
This optimization cannot be applied to non-array iterables since it relies
on passing indices and accessing an entry several times in some cases.
Hovewer, forcing inlining makes the code somewhat faster even for non-arrays.
Performance improvements are smaller but still significant for replace()
and skip(). There is 2× regression when passing a generator to fail(), though,
but the gain for the array case is worth it.

The second advantage of using custom iterators is that eltype() returns
Nulls.T(eltype(x)) when x is an array, while when using plain generators
it returned Any.