Union and SDiff instances for Std.RBSet

Stream: batteries

[thelissimus]

Hello. I was using Std.RBSet and couldn't find Union and SDiff instances, even though there are union and sdiff function implementations. Is there any reason that there are no instances (like API stability or something)? If not, I can make a PR, adding the instances. Figured, I would ask here beforehand, instead of creating redundant PRs and wasting reviewer's time.

[Ruben Van de Velde]

I think these classes only recently moved somewhere std can use them

[thelissimus]

I was guided by the instance of Union on the Std.List. Looking at git history, I was able to trace the addition of the instance to 2 years ago: commit.

So, is it okay if I make a PR?