gdb VecDeque pretty printer is incorrect

[artemmukhin]

As noted here, the VecDeque pretty-printer doesn't take into account the ring buffer structure.

Example:

use std::collections::VecDeque;

fn main() {
    let mut deque = VecDeque::new();
    for i in 1..8 {
        deque.push_back(i)
    }
    deque.pop_front();
    deque.push_back(8);
    println!("{:?}", deque);
}

Run gdb with b 10:

(gdb) p deque
$2 = VecDeque<i32>(len: -1, cap: 8)