Improve performance of .fold()

[jturner314]

This PR does two things that improve the performance of .fold() for ArrayBase:

1. Specialize the process of finding the inner axis for 2-D arrays.
2. Implement .fold() for the Axes iterator. This is an improvement for arrays with more than 2 axes.

For the function below with a 320x320 input array, this PR improves the performance by ~50%.

const RADIUS: usize = 1;
const WINDOW_SIZE: usize = 2 * RADIUS + 1;

fn sum_sq_diff_windows(data: ArrayView2<f64>) -> Array2<f64> {
    let mut out = Array2::zeros((data.rows() - 2 * RADIUS, data.cols() - 2 * RADIUS));
    Zip::from(&mut out)
        .and(data.windows((WINDOW_SIZE, WINDOW_SIZE)))
        .apply(|out, window| {
            let center = window[(RADIUS, RADIUS)];
            *out = window.fold(0., |acc, x| acc + (x - center).powi(2));
        });
    out
}

[bluss]

Nice. So it'a win even in a 3x3 array?

[jturner314]

So it'a win even in a 3x3 array?

Yes, each window is 3x3, and this PR significantly improves the performance because it reduces the cost of each .fold() call. (It doesn't improve the iteration performance within .fold(); it reduces the cost of determining the iteration order.)

[bluss]

Ah, of course. Thanks