A priority queue should guarantee order

`PriorityQueue` doesn't seem to guarantee order:

```ts
import { PriorityQueue } from 'typescript-collections';

interface Obj {
prio: number;
id: string;
}

const q = new PriorityQueue((a: Obj, b: Obj) => a.prio - b.prio);

q.enqueue({ prio: 0, id: 'first' });
q.enqueue({ prio: 10, id: 'prio' });
q.enqueue({ prio: 0, id: 'another 1' });
q.enqueue({ prio: 0, id: 'another 2' });
q.enqueue({ prio: 0, id: 'last' });

let o: Obj | undefined;
while (o = q.dequeue()) {
console.log(o.id);
}
```

outputs:

```
prio
last
another 2
another 1
first
```

(in this example the order is perfectly reversed but this won't be the case if `enqueue`/`dequeue` are interleaved)

Start at the PriorityQueue implementation and reproduce the TypeScript example from the issue. Inspect how equal-priority entries are stored and removed, then check whether the existing tests cover their ordering. Done means the queue's ordering for equal-priority items matches the project's intended guarantee and the reported reproduction is covered by a test.

Written by the indexing model from the issue text.