create a new `expanding_array` with element i set to v. Grow the array
in case i == length.

expand is not thread-safe.

Complexity: O(1) if no allocation is required, O(length+1) otherwise.

Cumulative complexity of adding an element to an empty `expanding_array`
and repeatedly growing the result `add` by a total of `n` elements is
`O(n)`.

`add` called repeatedly on the same `expanding_array` creates copies
of the underlying array data and hence has performance in `O(length)`.