Add the package to the table and start it off with a few small, basic
functions. Inspired by CL 66331, which added flag.ro.

Updates #21851.

Change-Id: I3995cde1ff7bb09a718110473bed8b193c2232a5
Reviewed-on: https://go-review.googlesource.com/66990
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>