Better representation of SLL head and tail

For single linked list it's more clear to rewrite this:
https://github.com/nitkach/data_structures/blob/main/src/bin/sll.rs#L5-L6
to something like this:
```rust
struct SLL {
    head_tail: Option<(Rc<RefCell<Node>>, Rc<RefCell<Node>>)>
}
```