unicode/norm: added NextBoundary(InString)

This turns out to result in much better code than
FirstBoundary, allows streaming, and is typically
more what users would want.

Change-Id: I40f4074c01d868080f336b9e466ba720e277cdcd
Reviewed-on: https://go-review.googlesource.com/24853
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>