derived Debug for Iter and IterMut

[biskwikman]

My attempt to implement Debug for Iter, I also did so for IterMut.
#1204

Example:

input:

fn main() {
    let foo = ndarray::Array1::<f64>::zeros(10);
    println!("{:?}", std::iter::zip(0..foo.len(), &foo));
}

output:

Zip { a: 0..10, b: Iter { inner: Slice(Iter([0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0])) } }

[nilgoyette]

Update the description with one output example. Also, you left several lines of commented code.

[nilgoyette]

I see 2 problems:

• Please also add the output of a 2D arrays so we know how it will be printed
• Iter { inner: Slice(Iter([...])) } } is uselessly descriptive. The user doesn't want to know the technical architecture of ndarray, he simply wants to print the values in the iterator. I think you will be forced to code the function instead of using #[derive(Debug)] in order to control what you print.

[bluss]

Rebased. It can be prettier and more logical but it's just a Debug fmt, so it can change later. Thanks, this is a good start for this.