Add lifetime-preserving version of iterators

[RReverser]

Currently, ArrayView has a helpful lifetime-preserving methods like to_slice, but no way to preserve lifetimes of iterators like axis_iter, outer_iter and such.

This makes it difficult to return iterables from helper functions like this:

fn lazy_process(&self) -> impl '_ + Serialize {
	IterSerialize(self.data.outer_iter().map(move |column| {
	    IterSerialize(column.outer_iter().map(|pixel| pixel.to_slice().unwrap())) // <-- cannot return value referencing function parameter `column`
	}))
}

I suggest there should be overrides like into_outer_iter, into_axis_iter etc. on ArrayView that would return iterators tied by the lifetime to the original data instead of the ArrayView itself.

Note: while looking for open issues, the only relevant I found was #320, but there the author could afford to just reshape / flatten the data; that might not always be possible, like in my example above. @bluss mentioned issues and suggestions similar to what I'm describing here though: #320 (comment)

[RReverser]

For the example above for now found a workaround like this but it's pretty ugly compared to what it could be:

IterSerialize(
    (0..self.data.len_of(ndarray::Axis(0))).map(move |column_i| {
        IterSerialize((0..self.data.len_of(ndarray::Axis(1))).map(move |row_i| {
            self.data
                .slice(ndarray::s![column_i, row_i, ..])
                .to_slice()
                .unwrap()
        }))
    }),
)

[akern40]

You're not gonna believe this... there is an into_axis_iter on the ArrayView types! It got listed as doc(hidden) in 2016 (6d33685) and shortly after as deprecated (5d98d80); I'm frankly not sure why. I'm happy to un-deprecate it and add the other .into_*_iter methods.

[RReverser]

Thank you!