(Although it's still half the speed of OpenSSL.)

benchmark           old ns/op    new ns/op    delta
BenchmarkRC4_128         1409          398  -71.75%
BenchmarkRC4_1K         10920         2898  -73.46%
BenchmarkRC4_8K        131323        23083  -82.42%

benchmark            old MB/s     new MB/s  speedup
BenchmarkRC4_128        90.83       321.43    3.54x
BenchmarkRC4_1K         93.77       353.28    3.77x
BenchmarkRC4_8K         61.65       350.73    5.69x

R=rsc, remyoudompheng
CC=golang-dev, jgrahamc
https://golang.org/cl/7234055