Go 1.10 is adding new API MarshalPKCS1PublicKey and
ParsePKCS1PublicKey for converting rsa.PublicKeys.

Even though we'd prefer that users did not, check that
if users call asn1.Marshal and asn1.Unmarshal directly instead,
they get the same results. We know that code exists in the
wild that depends on this.

Change-Id: Ia385d6954fda2eba7da228dc42f229b6839ef11e
Reviewed-on: https://go-review.googlesource.com/82080
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>