secure/precis: small optimization

Don't create set for each call to Transform.

Change-Id: I257f67e33aae21899dcc23a974a06e57d2258d91
Reviewed-on: https://go-review.googlesource.com/23594
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Sam Whited <sam@samwhited.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>