Simplify and speed up uniquify (issue 583390043 by address@hidden)

[dak]

Not sure about the speedup (we might have small sizes often enough that
the constant factor becomes relevant), but the code is quite a net win
with regard to obviously doing what it should do.  It probably takes a
whole lot more allocation calls, but at least the impact on memory is
just temporary.

LGTM

https://codereview.appspot.com/583390043/