This CL requires CL 5970055.

benchmark           old ns/op    new ns/op    delta
BenchmarkEncrypt          161           23  -85.71%
BenchmarkDecrypt          158           24  -84.24%
BenchmarkExpand           526           62  -88.21%

benchmark            old MB/s     new MB/s  speedup
BenchmarkEncrypt        99.32       696.19    7.01x
BenchmarkDecrypt       100.93       641.56    6.36x

R=golang-dev, bradfitz, dave, rsc
CC=golang-dev
https://golang.org/cl/6549055