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>