The Slicing-By-8 [1] algorithm has much performance improvements than
current approach. This patch only uses it for IEEE, which is the most
common case in practice.

There is the benchmark on Mac OS X 10.9:

benchmark                     old MB/s     new MB/s     speedup
BenchmarkIEEECrc1KB           349.40       353.03       1.01x
BenchmarkIEEECrc4KB           351.55       934.35       2.66x
BenchmarkCastagnoliCrc1KB     7037.58      7392.63      1.05x

This algorithm need 8K lookup table, so it's enabled only for block
larger than 4K.

We can see about 2.6x improvement for IEEE.

Change-Id: I7f786d20f0949245e4aa101d7921669f496ed0f7
Reviewed-on: https://go-review.googlesource.com/1863Reviewed-by: Russ Cox <rsc@golang.org>