array_at(a, i)
Return the element at the (zero-based) ith position of the array a.
The ith element can be also accessed using the array reference operator as, a[i].
Array_at and [] will raise an "Out of range" error if i < 0 or
i >= array_length(a).
let a = #[1, 2, "hi", 'abc]
array_at(a, 3)
// abc
a[2]
// hi