hash/crc64: Use slicing by 8.

hash/crc64: Use slicing by 8.

Similar to crc32 slicing by 8.
This also fixes a Crc64KB benchmark actually using 1024 bytes.

Crc64/ISO64KB-4       147µs ± 0%      37µs ± 0%   -75.05%  (p=0.000 n=18+18)
Crc64/ISO4KB-4       9.19µs ± 0%    2.33µs ± 0%   -74.70%  (p=0.000 n=19+20)
Crc64/ISO1KB-4       2.31µs ± 0%    0.60µs ± 0%   -73.81%  (p=0.000 n=19+15)
Crc64/ECMA64KB-4      147µs ± 0%      37µs ± 0%   -75.05%  (p=0.000 n=20+20)
Crc64/Random64KB-4    147µs ± 0%      41µs ± 0%   -72.17%  (p=0.000 n=20+18)
Crc64/Random16KB-4   36.7µs ± 0%    36.5µs ± 0%    -0.54%  (p=0.000 n=18+19)

name                old speed     new speed      delta
Crc64/ISO64KB-4     446MB/s ± 0%  1788MB/s ± 0%  +300.72%  (p=0.000 n=18+18)
Crc64/ISO4KB-4      446MB/s ± 0%  1761MB/s ± 0%  +295.20%  (p=0.000 n=18+20)
Crc64/ISO1KB-4      444MB/s ± 0%  1694MB/s ± 0%  +281.46%  (p=0.000 n=19+20)
Crc64/ECMA64KB-4    446MB/s ± 0%  1788MB/s ± 0%  +300.77%  (p=0.000 n=20+20)
Crc64/Random64KB-4  446MB/s ± 0%  1603MB/s ± 0%  +259.32%  (p=0.000 n=20+18)
Crc64/Random16KB-4  446MB/s ± 0%   448MB/s ± 0%    +0.54%  (p=0.000 n=18+20)

Change-Id: I1c7621d836c486d6bfc41dbe1ec2ff9ab11aedfc
Reviewed-on: https://go-review.googlesource.com/22222
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>