Implements PriorityTaskQueue with std's LinkedList rather than a custom linked list implementation

[michaelRichards99]

Fixes #377.

[michaelRichards99]

We tested with the existing micro benchmarks from the rusty-hermit project's benchmark directory.

With these changes, the scheduling time for 2 threads was an average of 2880 ticks.
With the custom implementation, it was an average of 2730 ticks.
This is only testing with 2 Tasks on the queue however, it would likely change with more tasks. We could write a benchmark for this if you would like to see more numbers.

Another potential issue with this implementation is that LinkedList's remove method is unstable, so we used a work around to avoid that.

[stlankes]

Looks good! But I will discuss it with @mkroening Just a double check.

[mkroening]

This looks good! Thanks a lot!

Two minor remarks, though the current code is definitely an improvement already. 👍

[michaelRichards99]

Thank you for the feedback :). We've addressed those 2 changes and made sure the priority bitmap is properly set when removing a task from a list.

Thank you @ynchen2829 for authoring these changes.

[mkroening]

Looks good to me!

Thanks a lot! :)

bors r+

[mkroening]

Ah, nice! You even made this a fully fledged method. 👍

[bors]

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