astutil: add Unparen utility, eliminating 7 copies.

Change-Id: I824328c275bd6198a57edd64bf72cb2cf0490a68
Reviewed-on: https://go-review.googlesource.com/2172
Reviewed-by: Chris Manghane <cmang@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>