Add .into_view/_mut() methods to Windows, ExactChunks, and ExactChunksMut

Locate Windows, ExactChunks, ExactChunksMut, and the DimAdd trait from issue #570. Compare the proposed Windows::into_view implementation with the corresponding types, then add the requested immutable and mutable view methods; done means all three types support the requested conversions while preserving outer and window axes.

This can be convenient for some use cases, such as copying windows (especially with a non-unit step between windows) into a new owned array for modification.

This would rely on the DimAdd trait from #570. For example, the implementation for Windows could be the following:

impl<'a, A, D: Dimension> Windows<'a, A, D> {
    /// Returns a view such that the outer axes are the iteration axes and the
    /// inner axes are the axes of the windows.
    ///
    /// # Example
    ///
    /// ```
    /// use ndarray::{array, aview2, s};
    ///
    /// let arr = array![
    ///     [0, 1, 2, 3],
    ///     [4, 5, 6, 7],
    ///     [8, 9, 10, 11],
    /// ];
    /// let windows = arr.windows([2, 3]).into_view();
    /// assert_eq!(windows.shape(), &[2, 2, 2, 3]);
    /// assert_eq!(
    ///     windows.slice(s![0, 0, .., ..]),
    ///     aview2(&[[0, 1, 2], [4, 5, 6]]),
    /// );
    /// assert_eq!(
    ///     windows.slice(s![0, 1, .., ..]),
    ///     aview2(&[[1, 2, 3], [5, 6, 7]]),
    /// );
    /// assert_eq!(
    ///     windows.slice(s![1, 0, .., ..]),
    ///     aview2(&[[4, 5, 6], [8, 9, 10]]),
    /// );
    /// assert_eq!(
    ///     windows.slice(s![1, 1, .., ..]),
    ///     aview2(&[[5, 6, 7], [9, 10, 11]]),
    /// );
    /// ```
    pub fn into_view(self) -> ArrayView<'a, A, <D as DimAdd<D>>::Output> {
        let base_ndim = self.base.ndim();
        let out_ndim = 2 * base_ndim; // check for overflow?
        let mut out_dim = <D as DimAdd<D>>::Output::zeros(out_ndim);
        out_dim.slice_mut()[..base_ndim].copy_from_slice(&self.base.dim.slice());
        out_dim.slice_mut()[base_ndim..].copy_from_slice(&self.window.slice());
        let mut out_strides = <D as DimAdd<D>>::Output::zeros(out_ndim);
        out_strides.slice_mut()[..base_ndim].copy_from_slice(&self.base.strides.slice());
        out_strides.slice_mut()[base_ndim..].copy_from_slice(&self.strides.slice());
        unsafe { self.base.with_strides_dim(out_strides, out_dim) }
    }
}