Replace linked list with VecDeque

[Igigog]

Hey :)
I was looking around out of curiosity and found a nice place to reduce the amount of unsafety. This PR replaces Linked List-y structure of JobQueue with Rust's own VecDeque. As far as I can tell, there is no difference in performance (~1% difference on my machine). Also:

1. It is easier to read.
2. Tests for the queue are not needed anymore, since there is no reason to test VecDeque.
3. Since there is no sentinel value, pop_back and pop_front are safe.

[dragostis]

Thank you for the PR! :)

I actually started writing this project with VecDeque, but I was actually pushing the whole Job onto the queue instead of the pointer which was slower. Nice to see that it actually works!

[dragostis]

Looks pretty good, but there are a few things that can be further simplified before we can merge this. I would also keep the pop_* methods unsafe.

[Igigog]

Indeed, using fut as a queue marker made things quite a bit easier and ~2% faster on my machine, which makes this PR at least not slower than main branch. I have renamed is_in_queue method to is_waiting, since it doesn't really show if it's in queue anymore.

As for pop_* methods, pop_back is now very obviously safe; I maintain that pop_front is not unsafe, but I don't really care tbh, might as well mark it unsafe.

[dragostis]

Thanks for the fixes! Ofc, the pop methods don't need to be unsafe.