Auto merge of #53564 - MaloJaffre:vecdeque, r=gnzlbg

Reoptimize VecDeque::append

~Unfortunately, I don't know if these changes fix the unsoundness mentioned in #53529, so it is stil a WIP.
This is also completely untested.
The VecDeque code contains other unsound code: one example : [reading unitialized memory](https://play.rust-lang.org/?gist=6ff47551769af61fd8adc45c44010887&version=nightly&mode=release&edition=2015) (detected by MIRI), so I think this code will need a bigger refactor to make it clearer and safer.~

Note: this is based on #53571.
r? @SimonSapin
Cc: #53529 #52553 @YorickPeterse @jonas-schievink @Pazzaz @shepmaster.