x/crypto/*: add import comment Add import comment for blake2b, blake2s, chacha20poly1305 and cryptobyte. Change-Id: I4703b5cd669e43a5d81422b2ded8b8f54eee5f9b Reviewed-on: https://go-review.googlesource.com/39952 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>