utils/data: Use list comprehension for uniq_stable()

This should be faster than and explicit for loop.  Also, a set makes more
sense than a dict if we only care about values.  After these changes, the
resulting code is the same as Dave Kirby's uniqify() suggestion on Peter
Bengtsson's blog [1].

[1]: http://www.peterbe.com/plog/uniqifiers-benchmark