internal/number: add number formatting Now supports formatting of decimal, scientific/ engineering and percent. Change-Id: If94b0684dc1b1bd66106a07828d3b7f695f12e2d Reviewed-on: https://go-review.googlesource.com/45494 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>