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>
5 files changed