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