internal/number: move all rounding data in one type

Change-Id: If5752ddeee6224a347b4feccd0195f1057bd5397
Reviewed-on: https://go-review.googlesource.com/58410
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
6 files changed