This exposes the chacha20 stream cipher to the entire x/crypto package, and in particular to the SSH package, which uses separate, unauthenticated chacha20 encryption for packet lengths. Change-Id: I0b705482128f0657c09292370f03d08b588f7fec Reviewed-on: https://go-review.googlesource.com/87075 Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>