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>
2 files changed