feature/plural: pull in Form from internal plural package Change-Id: Ib82046f65688dee8a244eaada03c6a9193856c5b Reviewed-on: https://go-review.googlesource.com/39134 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>