unicode/rangetable: added New and Visit

I find myself wanting these utilities in testing code and they
seem to be generally useful enough to make public.

Change-Id: I4afd7cfc99bc797c5c12fe12bad40f0f16ed3400
Reviewed-on: https://go-review.googlesource.com/11493
Reviewed-by: Nigel Tao <nigeltao@golang.org>
4 files changed