This commit improves the processing of the final few bytes in
castagnoliSSE42: instead of processing one byte at a time, we use all
versions of the CRC32 instruction to process 4 bytes, then 2, then 1.
The difference is only noticeable for small "odd" sized buffers.

We do the similar improvement for processing the first few bytes in
the case of unaligned buffer.

Fixing the test which was not actually verifying the results for
misaligned buffers (WriteString was creating an internal copy which
was aligned).

Adding benchmarks for length 15 (aligned and misaligned), results
below.

name                          old time/op    new time/op    delta
CastagnoliCrc15B-4              25.1ns ± 0%    22.1ns ± 1%  -12.14%
CastagnoliCrc15BMisaligned-4    25.2ns ± 0%    22.9ns ± 1%   -9.03%
CastagnoliCrc40B-4              23.1ns ± 0%    23.4ns ± 0%   +1.08%
CastagnoliCrc1KB-4               127ns ± 0%     128ns ± 0%   +1.18%
CastagnoliCrc4KB-4               462ns ± 0%     464ns ± 0%     ~
CastagnoliCrc32KB-4             3.58µs ± 0%    3.60µs ± 0%   +0.58%

name                          old speed      new speed      delta
CastagnoliCrc15B-4             597MB/s ± 0%   679MB/s ± 1%  +13.77%
CastagnoliCrc15BMisaligned-4   596MB/s ± 0%   655MB/s ± 1%   +9.94%
CastagnoliCrc40B-4            1.73GB/s ± 0%  1.71GB/s ± 0%   -1.14%
CastagnoliCrc1KB-4            8.01GB/s ± 0%  7.93GB/s ± 1%   -1.06%
CastagnoliCrc4KB-4            8.86GB/s ± 0%  8.83GB/s ± 0%     ~
CastagnoliCrc32KB-4           9.14GB/s ± 0%  9.09GB/s ± 0%   -0.58%

Change-Id: I499e37af2241d28e3e5d522bbab836c1a718430a
Reviewed-on: https://go-review.googlesource.com/24470Reviewed-by: Keith Randall <khr@golang.org>