Make change_priority API consistent

[liwenjieQu]

Currently for either PriorityQueue or DoublePriorityQueue, the return value for change_priority and change_priority_by functions are different.

In some cases where a customized (and complicated) struct P is used, change_priority_by is usually preferred which provides flexibility to update the underlying structure. However, due to nothing is returned, the caller has no idea whether the update is successful or not. There are few options

1. make change_priority_by return an Option type (same as change_priority
2. Call get_priority first. Based on result and then call change_priority_by
3. Call push

2 and 3 are obviously not ideal, as Option2 caused IndexMap lookup twice; Option3 may cause a clone
This PR submits code change for Option1

[garro95]

I would rather return a bool then Option<()>.
Keep in mind that, anyway, the user could already use std::mem::replace, std::mem::swap or the unsafe take_mut crate to take out an Option(p1), where p1 is the old priority. For example, you could do something like

use priority_queue::PriorityQueue;
use std::mem::swap;

let mut pq = PriorityQueue::new();

pq.push("Apples", 5);
pq.push("Bananas", 8);
pq.push("Strawberries", 23);

let mut newp = Some(10);

pq.change_priority_by("Bananas", |p| {
  swap(p, newp.as_mut().unwrap());
});
let oldp = newp;
assert_eq!(pq.get("Bananas"), Some((&"Bananas", &10)));
assert_eq!(oldp, Some(8));

[liwenjieQu]

Make sense to me. I will update the PR :-)

Either bool or Option will work for my case. The old value can be fetched in a way as you suggested.

[garro95]

Would you also be so kind to update the change_priority_by tests to check that the return value is always correct?

[garro95]

Thanks for your contribution