Rollup merge of #123947 - zopsicle:vec_deque-Iter-as_slices, r=Amanieu

Add vec_deque::Iter::as_slices and friends

Add the following methods, that work similarly to VecDeque::as_slices:

 - alloc::collections::vec_deque::Iter::as_slices
 - alloc::collections::vec_deque::IterMut::into_slices
 - alloc::collections::vec_deque::IterMut::as_slices
 - alloc::collections::vec_deque::IterMut::as_mut_slices

Obtaining slices from a VecDeque iterator was not previously possible.

Author: matthiaskrgr