Add `params` overloads

[JamesFaix]

For some operators, it would be very convenient to have overloads that take params arrays as arguments.

• Union(this IEnumerable<T> source, T item, params T[] items)
• Except(this IEnumerable<T> source, T item, params T[] items)
• Concat(this IEnumerable<T> source, T item, params T[] items)
• Intersect(this IEnumerable<T> source, T item, params T[] items)

Intersect seems less useful than the others, but it might as well be included for completeness if the other three are.

[atifaziz]

This should be put on hold as dotnet/roslyn#36 will help to avoid adding all those overloads (and more) if the feature makes it into C# 7 or vNext. Meanwhile, it's merely about convenience for very short and hard-coded sequences.

[JamesFaix]

That's understandable.

[SergeyZhurikhin]

@JamesFaix vote!

https://visualstudio.uservoice.com/forums/121579-visual-studio-2015/suggestions/3148411--params-keyword-for-every-list

[JamesFaix]

Thanks Sergey. Of all the suggestions I saw on there though, I think params enhancements is a pretty trivial matter. (But, perhaps that will make it more likely to be implemented.) All my votes are currently on suggestions dealing with static interfaces/generics, integrated code contracts, and non-nullable types.