Add a pop_lowest_priority API

[Luni-4]

It would be helpful to add a pop API that extracts the lowest priority from the queue and not only the greatest one (as documented here)

[garro95]

If you need a PriorityQueue that can extract at any time the greatest or the lowest priority, you can use the DoublePriorityQueue data structure. It features two methods: pop_max() and pop_min().

If you always want to extract the minimum priority item, you can wrap your priorities in a standard wrapper called Reverse<T>

[Luni-4]

Oh, thank you! Could you please provide some examples in README or inside the examples directory about these two patterns you just mentioned? That would be really helpful! Thank you!

[rikonor]

@Luni-4 I needed this too - here's an example:

let mut pq: PriorityQueue<String, Reverse<u64>> = PriorityQueue::new();

pq.push("1".into(), Reverse(10));
pq.push("2".into(), Reverse(15));