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>
4 files changed