secure/precis: fix compare order of operations

Reduce the number of heap allocations required by the compare operation,
and factor out enforce and compare transformations into a single
function.

Also throw in a minor typo fix in a comment.

Change-Id: I8e9fec1fc51d38a1950f95be72e07fb80d9949e3
Reviewed-on: https://go-review.googlesource.com/29966
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>