go.crypto/sha3: optimize round function
go.crypto/sha3: optimize round function

benchmark                        old ns/op     new ns/op     delta
BenchmarkPermutationFunction     1128          733           -35.02%
BenchmarkShake256_1MiB           10004381      6832715       -31.70%

benchmark                        old MB/s     new MB/s     speedup
BenchmarkPermutationFunction     177.23       272.54       1.54x
BenchmarkShake256_1MiB           104.81       153.46       1.46x

LGTM=agl
R=golang-codereviews, agl
CC=golang-codereviews
https://codereview.appspot.com/156770043

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

isislovecruft/go.crypto,isislovecruft/go.crypto