Add support for converting a HashSet to an Array or a Slice

[tur11ng]

I think this will be a very handful feature and such a method is really common and is provided by other languages such as Java or Python for example. Let me know if you need any help with the implementation

[tur11ng]

For example such a method would be really helpful, myCustomSet.AsSlice(). But I see that you have implemented the HashSet using a HashMap and the latter uses a continuous array for the Entry values not two separate ones, one for the Key and one for the Values. You implementations performs better for looping through the entries as a whole but it doesn't provide a direct efficient way of accessing the keys or the values.