Inconsistency in *_mut functions

[msrd0]

Right now, PriorityQueue::get_mut does not allow modifying the item of the queue (which is in line with the API of other maps in the rust ecosystem) but PriorityQueue::iter_mut does which is both inconsistent with the get_mut function and inconsistent with the API of other maps in the rust ecosystem.

I propose to add PriorityQueue::get_mut2 or PriorityQueue::get_full_mut2 and PriorityQueue::iter_mut2 similar to how the underlying IndexMap works and mark PriorityQueue::iter_mut as deprecated. Then, in the next major version, I propose to replace iter_mut with an iterator that does not allow modifying the item but just the priority.

The same is true for the DoublePriorityQueue.

If you agree to this change, I can send a PR if you want.

Btw for the motivation: My items have a "parent" that is not part of the eq/hash implementation but needs to be updated when changing the priority of the item as well, so I need the get_mut2 functionality. The alternative would be interior mutability which I'd rather not use, or iterating through the entire q to change one single element, which I'd also rather not do.