Rollup merge of #58064 - llogiq:vec-deque-try-rfold, r=scottmcm

Rollup merge of #58064 - llogiq:vec-deque-try-rfold, r=scottmcm

override `VecDeque::try_rfold`, also update iterator

This keeps the slice based iteration and updates the iterator state after each slice. It also uses a loop to reduce the amount of code.

This uses unsafe code, so some thorough review would be appreciated. Cc @RalfJung