Adding swap to Data.Tuple

[roconnor at theorem.ca]

Reopening discussion.

I believe that swap should be lazy by default in order to be consistent 
with how uncurry works.

-- 
Russell O'Connor                                      <http://r6.ca/>
``All talk about `theft,''' the general counsel of the American Graphophone
Company wrote, ``is the merest claptrap, for there exists no property in
ideas musical, literary or artistic, except as defined by statute.''