Add DList examples

Someone rightfully complained in IRC that DList was lacking examples. Here are some.
I think we can assume from context that this is already in scope. I don't see the benefit in repeating it in every example.

Of course, the examples still need to compile, but you can suppress it from showing up in the rendered results by prefixing the line with `#`, as in

```rust
/// ```rust
/// # use std::collections::dlist::DList;
/// # use std::collections::Deque;
```