TODO List for Addressable Pairing Heap
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- Implement `PairingHeap::get_key(h: Handle) -> K` method to retrieve the key associated with the element that is associated with the given handle `h`.

- Find a solution to efficiently support merging of `PairingHeap` instances.
- Find a better API for `decrease_key`. Maybe `set_key` which is more efficient for lowering?
- Improve docs with code examples.
- Add benchmarks:
   - To compare performance between `SmallVec` and `Vec` for children storage in `Node`.
   - To compare performance between `PairingHeap` and `BinaryHeap` of the standard library.