This patch introduces specialized functions for initial
and final permutations, and precomputes the output of the
third permutation on the S-box elements.

benchmark           old ns/op    new ns/op    delta
BenchmarkEncrypt         3581         1226  -65.76%
BenchmarkDecrypt         3590         1224  -65.91%

benchmark            old MB/s     new MB/s  speedup
BenchmarkEncrypt         2.23         6.52    2.92x
BenchmarkDecrypt         2.23         6.53    2.93x

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12072045