unicode/precis: Add PRECIS opaque string profile

Adds an implementation of the PRECIS opaque string profile as defined in
RFC 7613.

Change-Id: I26861f701e0e39958ee0710db727790008598eb4
Reviewed-on: https://go-review.googlesource.com/19090
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>