internal/number: fix rounding issues

- rounding with large scales
- correct interpretation for engineering
- default rounding is exact representation
- simplify increments

Change-Id: I6a4959974069f5e1706e396a836bd6989b93d60d
Reviewed-on: https://go-review.googlesource.com/60350
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
8 files changed