secure/precis: add lookahead for context rules

Also completes all the tests for context rules.

Could not measure a noticeable slowdown for the
benchmarks.

Change-Id: I3ec20b3b0fed0c52764f02bc744cecae1d745123
Reviewed-on: https://go-review.googlesource.com/23872
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed