internal/number: separate scale type

Scale of number may differ from scale of
increment. This is rare, but separating
concerns simplifies code down the line
and prevents bugs.

Change-Id: Icc5d9afe984f54875967033ed673a7d5fe6dd1fa
Reviewed-on: https://go-review.googlesource.com/60370
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>