`map_windows` without const-generics?

[Philippe-Cholet]

As of today, Iterator::map_windows is nightly:

fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>
where
    Self: Sized,
    F: FnMut(&[Self::Item; N]) -> R;

But I'm wondering if we should have a version of it without const-generics (so a boxed slice internally) as it would allow to have the feature with a n unknown at compile time, similar to slice.windows(n).map(f).

#[cfg(feature = "use_alloc")]
fn map_boxed_windows<F, R>(self, n: usize, f: F) -> MapBoxedWindows<Self, F>    // or any other names
where
    Self: Sized,
    F: FnMut(&[Self::Item]) -> R;

• It can have a very similar internal implementation: a boxed slice of length 2 * n to reduce moves (move all windows' elements one time each n elements).
• It could have a boxed slice of minimal length n (to minimize allocation) at the cost of moving all the windows' elements every time (with .rotate_left(1)).

Both allocate once. The first one would be my choice but the second would be simpler and easier to review.

We could also wait for lending iterators to discard F entirely, but it's gonna be a long wait.