Use UniqueStash instead of Stash

[FedericoStra]

I think it would be safer to use UniqueStash instead of Stash in order to prevent decreasing an unwanted item with an outdated Handle. Consider the following example:

let mut heap = DefaultPairingHeap::new();
let h1 = heap.push('a', 1);
heap.pop(); // we pop the item `h1` was referring to
let _h2 = heap.push('b', 1);
heap.decrease_key(h1, 0); // here `h1` should no longer be valid
println!("{:?}", heap)

which outputs

PairingHeap { min: Handle(0), roots: [Handle(0)],
    data: {0: Node { pos: Root(0), entry: Entry { key: 0, elem: 'b' }, children: [] }} }

It shows that we have actually accidentally decreased the element indexed by h2, and not the the no longer existing item 'a', which originally gave us the handle.

I think the current implementation breaks the usual guarantees of rust, namely that references should not accidentally change the object they are referring to (and yes, the Handle plays the role of a reference; it's only a trick to circumvent the impossibility of using an actual reference)