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>
4 files changed