subarray_fill(a, start, end, obj)
Set the elements of the array a, beginning with index start (inclusive) and ending
with index end (exclusive) to the object obj.
Return void.
let a = #[1, 2, 3, 4, 5]
subarray_fill(a, 2, 4, 'x)
a
// #[1, 2, x, x, 5]