Allow `slice_axis` slicing with the `s!` macro

[bokutotu]

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Hello! I would like to thank you for developing this wonderful repository.

I think it would be useful to be able to index the slice_axis,slice_axis_mut,slice_axis_inplace methods with s! macro. What do you think?

I think there is a way to implement it as follows. If this method is good, I would like to make a PR.

slice.rs

impl<Din, Dout> From<SliceInfo<[SliceInfoElem; 1], Din, Dout>> for Slice
where
    Din: Dimension,
    Dout: Dimension,
{
    fn from(index: SliceInfo<[SliceInfoElem; 1], Din, Dout>) -> Slice {
        match index.deref()[0] {
            SliceInfoElem::Slice { start, end, step } => Slice::new(start, end, step),
            SliceInfoElem::Index(index) => Slice::new(index, Some(index + 1), 1),
            SliceInfoElem::NewAxis => {
                panic!("can not convert to Slice")
            }
        }
    }
}

impl_method.rs

    pub fn slice_axis<I>(&self, axis: Axis, indices: I) -> ArrayView<'_, A, D>
    where
        S: Data,
        I: Into<Slice>,
    {
        let mut view = self.view();
        view.slice_axis_inplace(axis, indices);
        view
    }

    pub fn slice_axis_mut<I>(&mut self, axis: Axis, indices: I) -> ArrayViewMut<'_, A, D>
    where
        S: DataMut,
        I: Into<Slice>,
    {
        let mut view_mut = self.view_mut();
        view_mut.slice_axis_inplace(axis, indices);
        view_mut
    }
    pub fn slice_axis_inplace<I>(&mut self, axis: Axis, indices: I)
    where
        I: Into<Slice>,
    {
        let offset = do_slice(
            &mut self.dim.slice_mut()[axis.index()],
            &mut self.strides.slice_mut()[axis.index()],
            indices.into(),
        );
        unsafe {
            self.ptr = self.ptr.offset(offset);
        }
        debug_assert!(self.pointer_is_inbounds());
    }