R=golang-dev, bradfitz CC=golang-dev https://golang.org/cl/6827061
Fixes golang/go#4102. R=russross, minux.ma, rsc, agl CC=golang-dev https://golang.org/cl/6623053
Package salsa20 implements the Salsa20 stream cipher as specified in http://cr.yp.to/snuffle/spec.pdf. (Reference implementation by dchest.) R=golang-dev, dchest CC=golang-dev https://golang.org/cl/6496083