Consider making `Collection::Item` `?Sized`

[apasel422]

This would enable node-based collections to directly embed trait objects. For example:

struct Node<T: ?Sized> {
    left: Option<Box<Node<T>>>,
    right: Option<Box<Node<T>>>,
    item: T,
}

pub struct Heap<T: ?Sized> {
    root: Option<Box<Node<T>>>,
}