Consider adding support for matrix rows as iterables in list comprehension

It may make sense to add support for matrix rows as iterables in list comprehension. For e.g.

```
let a = [[1, 2, 3, 4], [5, 6, 7, 8]]
let b = [c for c in a[1]]
let d = [e for e in a[1][1..3]]
```