Add doc that explains how to mutate IArray

[cecton]

Closes #17
Closes #34

[kirillsemyonkin]

Actually now that I think about it, it should be the only proper way. The Vec is just that optimal for making size-changing mutations (automatic reallocations and whatnot), it does not make sense to try to introduce some custom workaround that is definitely worse. For non-size-changing operations, make_mut already exists.

I got reminded that the Single variant prevents recursive structures:

#[derive(Clone)]
struct Node {
    children: IArray<Node>, // error: infinite size, because of the [Node; 1] variant
    // use `children: Rc<IArray<Node>>,` instead
}

impl ImplicitClone for Node {}

There probably should be an issue.