internal/number: introduce Digits Digits are needed by pluralization. add setScale, update, setPrecision Change-Id: If7ac134e8371f232421e69c1abe58a52296b5948 Reviewed-on: https://go-review.googlesource.com/58550 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>