pub fn cursor_back_mut(&mut self) -> CursorMut<'_, T> {
        CursorMut { index: self.len.checked_sub(1).unwrap_or(0), current: self.tail, list: self }
    }
Provides a reference to the front element, or `None` if the list is
empty.

# Examples

```
use std::collections::LinkedList;

let mut dl = LinkedList::new();
assert_eq!(dl.front(), None);
benchmarks/haystacks/code/rust-library.rs