By switching the lists I was using as stacks out with a deque, I
appear to have achieved a teeeensy speed improvement.
List `append()` and `pop()` methods have at most a O(n) time
complexity, where deques have a O(1) time complexity.
https://docs.python.org/3/library/collections.html#collections.deque
Signed-off-by: AKU <tom@tdpain.net>
