This change imports the curve25519 package from x/crypto at revision
594708b89f21ece706681be23d04a6513a22de6e.

Change-Id: I379eaa71492959e404259fc1273d0057573bc243
Reviewed-on: https://go-review.googlesource.com/30822Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>