Align `Set` with JavaScript `Set`

[takameyer]

Currently, the Set does not perfectly reflect a JS Set. Set currently extends an OrderedCollection, which implements a ReadOnlyArray.

• keys should return values (Sets aren't syntactically index accessible)
• Set is not ordered, so extending OrderedCollection may lead to assumptions that the Set is ordered.
• May be worth looking into Set implementing a JS Set, rather than extending OrderedCollection

[kraenhansen]

Our RealmSet extends the OrderedCollection which implements these for it?

[takameyer]

@kraenhansen Good point. I'll have to write some tests as none of this is covered. The implementation of keys does not match a JS Set, which is the same result as values.

[takameyer]

I have created test cases for this feature.

[takameyer]

@kraenhansen I've refactored the feature request. This would be a breaking change, but probably not high value.