secure/precis: fix lookahead code

Rewrote code to make it clearer and
remove the funkiness.

Change-Id: I603afef758cac0c2cd95999d0367fbd1e243e280
Reviewed-on: https://go-review.googlesource.com/44954
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Sam Whited <sam@samwhited.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>