containers: symmetricDifference for Set and HashSet

[Ben Franksen]

Would it make sense to add a function like the following to containers:

  symmetricDifference :: Set a -> Set a -> (Set a, Set a, Set a)
  symmetricDifference a b = (a \\ b, a `intersection` b, b \\ a)

with the idea that this can be implemented more efficiently as a
primitive than the above specification? (And similarly for HashSet.)

If this is the case, then perhaps difference and intersection could be
defined in terms of this new primitive:

  difference a b = let (r, _, _) = symmetricDifference in r
  intersection a b = let (_, r, _) = symmetricDifference in r

*if* it turns out that this does not degrade performance.

Cheers
Ben