feature/plural: compute valid forms for a language Needed for plural.Select. Change-Id: I52ccca45ec3622d7ac000da021a16b1efcdf08ba Reviewed-on: https://go-review.googlesource.com/41090 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>