`ArrayWindows<I; N>` should be `ExactSizeIterator` when `I` is and `N > 0`

[ronnodas]

If iter yields k items then iter.array_windows::<N>() yields (k + 1).saturating_sub(N) items. While the condition N > 0 is currently not enforceable via where bounds, we can have the impl concretely for N in 1..=12.