fn is_contiguous(&self) -> bool {
        // FIXME: Should we consider `head == 0` to mean
        // that `self` is contiguous?
        self.tail <= self.head
    }
Inserts an element at `index` within the `VecDeque`, shifting all elements with indices
greater than or equal to `index` towards the back.

Element at index 0 is the front of the queue.

# Panics

Panics if `index` is greater than `VecDeque`'s length

# Examples

```
use std::collections::VecDeque;

let mut vec_deque = VecDeque::new();
vec_deque.push_back('a');
vec_deque.push_back('b');
benchmarks/haystacks/code/rust-library.rs