util: make WakeList::wake_all use FIFO ordering

[paolobarbolini]

Motivation

#6517 (comment) noted that the current implementation of WakeList::wake_all is LIFO, while the original pre-WakeList implementations were FIFO. The very first implementation of WakeList was FIFO, but it also was also unsound in the case the Waker panicked, so it was changed to work as it is currently implemented.

Solution

This changes WakeList to be FIFO by processing Wakers in order and using a DropGuard to handle panics.

[Darksonn]

Looks correct, but I have some comments on the safety comments.

[Darksonn]

Thanks.