This adds "slicing by 8" optimization to Castagnoli tables which will
speed up CRC32 calculation on systems without asssembler,
which are all but AMD64.

In my tests, it is faster to use "slicing by 8" for sizes all down to
16 bytes, so the switchover point has been adjusted.

There are no benchmarks for small sizes, so I have added one for 40 bytes,
as well as one for bigger sizes (32KB).

Castagnoli, No assembler, 40 Byte payload: (before, after)
BenchmarkCastagnoli40B-4   10000000     161 ns/op         246.94 MB/s
BenchmarkCastagnoli40B-4   20000000     100 ns/op         398.01 MB/s

Castagnoli, No assembler, 32KB payload: (before, after)
BenchmarkCastagnoli32KB-4     10000     115426 ns/op      283.89 MB/s
BenchmarkCastagnoli32KB-4     30000     45171 ns/op       725.41 MB/s

IEEE, No assembler, 1KB payload: (before, after)
BenchmarkCrc1KB-4       500000     3604 ns/op         284.10 MB/s
BenchmarkCrc1KB-4      1000000     1463 ns/op         699.79 MB/s

Compared:
benchmark                     old ns/op     new ns/op     delta
BenchmarkCastagnoli40B-4      161           100           -37.89%
BenchmarkCastagnoli32KB-4     115426        45171         -60.87%
BenchmarkCrc1KB-4             3604          1463          -59.41%

benchmark                     old MB/s     new MB/s     speedup
BenchmarkCastagnoli40B-4      246.94       398.01       1.61x
BenchmarkCastagnoli32KB-4     283.89       725.41       2.56x
BenchmarkCrc1KB-4             284.10       699.79       2.46x

Change-Id: I303e4ec84e8d4dafd057d64c0e43deb2b498e968
Reviewed-on: https://go-review.googlesource.com/19335
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>