secure/precis: Add more benchmarks

Add some simple benchmarks for each profile for the purposes of
optimizing the allocations per operation.

Change-Id: I1b0e8812ca5b2e3ca72948c5404d569879e54fd2
Reviewed-on: https://go-review.googlesource.com/19251
Reviewed-by: Nigel Tao <nigeltao@golang.org>