ssh: add IUTF8 constant from RFC 8160
ssh: add IUTF8 constant from RFC 8160

Change-Id: I65ac40d86c4f0ee5552cd2e167ac91a735f6c56e
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/392134
Trust: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Maisem Ali <7c693fc9861ca64bf2c3972e928f93902ea46711@tailscale.com>

golang/crypto,golang/crypto,golang/crypto