[IndexedDB] Introduce IDBKeySet (#19)

To allow getAll() (and friends) to retrieve a disjoint set of keys, introduce `IDBKeySet` which can be used for any of the get-type methods (`get`, `getKey`, `getAll`, `getAllKeys`) instead of a key-or-key-range.

```webidl
[Constructor(sequence<any>), Exposed=(Window,Worker)]
partial interface IDBKeySet {
  readonly setlike<any>;
};
```

... hrm, okay, now that seems silly. Why not just an ES6 Set where non-keys result in failure? Need to discuss with others...

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/19