internal/number: clean up RoundingContext

- remove Scale and Precision fields
- pass RoundingContext as value (safer)
- fixes a bug as a side-effect

Change-Id: I248eccae2ad7fa5fe411b0893eaf784e8b10aca7
Reviewed-on: https://go-review.googlesource.com/58570
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