See discussion at #1438https://github.com/leanprover/lean/pull/1438#discussion_r105007325
@digama0 With this commit, the original `array_list.write` will also
perform a destructive update when the reference counter for `l` is 1.
```lean def write {α} (l : array_list α) : fin l^.length → α → array_list α :=
λ ⟨n, h⟩ v, { l with data := l^.data^.write ⟨n, l^.lt_capacity h⟩ h v }
```