Auto merge of #106241 - Sp00ph:vec_deque_iter_methods, r=the8472

Implement more methods for `vec_deque::IntoIter`

This implements a couple `Iterator` methods on `vec_deque::IntoIter` (`(try_)fold`, `(try_)rfold` `advance_(back_)by`, `next_chunk`, `count` and `last`) to allow these to be more efficient than their default implementations, also allowing many other `Iterator` methods that use these under the hood to take advantage of these manual implementations. `vec::IntoIter` has similar implementations for many of these methods. This PR does not yet implement `TrustedRandomAccess` and friends, as I'm not very familiar with the required safety guarantees.

r? `@the8472` (since you also took over my last PR)

Author: bors