Add tests to ensure that the size of important types don't change
unexpectedly.

Skip the test on nacl platforms because of their unusual padding
requirements.

Change-Id: Iddb127a99499e089a309b721f5073356c0da8b24
Reviewed-on: https://go-review.googlesource.com/20285Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>