internal/number: factor out code

for implementers of Convert (down the line)

removed scale and precision while at it

Change-Id: I0a7ef49d63e7042cd8b3602d56dd3cc472ad328b
Reviewed-on: https://go-review.googlesource.com/60710
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>