unicode/norm: implement Spanner interface Change-Id: I5e252a2d4dd82ccd9d2a3cfb62cf6916a488dc53 Reviewed-on: https://go-review.googlesource.com/28132 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>