(up to 2.8x).

This is a partially unrolled version which performs better for small
hashes and only sacrifices a small amount of ultimate speed to a fully
unrolled version which uses 5k of code.

Code size

  Before 1636 bytes
  After 1880 bytes
  15% larger

Benchmarks on Samsung Exynos 5 ARMv7 Chromebook

benchmark              old ns/op    new ns/op    delta
BenchmarkHash8Bytes         1907         1136  -40.43%
BenchmarkHash1K            20280         7547  -62.79%
BenchmarkHash8K           148469        52576  -64.59%

benchmark               old MB/s     new MB/s  speedup
BenchmarkHash8Bytes         4.19         7.04    1.68x
BenchmarkHash1K            50.49       135.68    2.69x
BenchmarkHash8K            55.18       155.81    2.82x

LGTM=dave, agl
R=dave, bradfitz, agl, adg, nick
CC=golang-codereviews
https://golang.org/cl/56990044