zstd: Improve "best" compression (#677)

* [zstd/enc] Cache align struct for big perf boost

```
benchstat -delta-test none old2.txt new2.txt
name                                  old time/op    new time/op     delta
Encoder_EncodeAllXML-16                 13.3ms ± 0%     13.0ms ± 0%    -2.11%
Encoder_EncodeAllSimple/fastest-16       229µs ± 0%      227µs ± 0%    -1.08%
Encoder_EncodeAllSimple/default-16       343µs ± 0%      371µs ± 0%    +8.13%
Encoder_EncodeAllSimple/better-16        402µs ± 0%      393µs ± 0%    -2.33%
Encoder_EncodeAllSimple/best-16         6.41ms ± 0%     2.72ms ± 0%   -57.48%  <====
Encoder_EncodeAllSimple4K/fastest-16    2.70µs ± 0%     2.56µs ± 0%    -5.26%
Encoder_EncodeAllSimple4K/default-16    33.1µs ± 0%     33.5µs ± 0%    +1.30%
Encoder_EncodeAllSimple4K/better-16     39.3µs ± 0%     38.8µs ± 0%    -1.12%
Encoder_EncodeAllSimple4K/best-16        732µs ± 0%      360µs ± 0%   -50.90%   <====
Encoder_EncodeAllHTML-16                 213µs ± 0%      209µs ± 0%    -2.07%
Encoder_EncodeAllTwain-16               3.23ms ± 0%     3.23ms ± 0%    -0.04%
Encoder_EncodeAllPi-16                  1.12ms ± 0%     1.11ms ± 0%    -1.01%
Random4KEncodeAllFastest-16              988ns ± 0%      976ns ± 0%    -1.31%
Random10MBEncodeAllFastest-16           2.50ms ± 0%     2.48ms ± 0%    -0.70%
Random4KEncodeAllDefault-16             4.58µs ± 0%     4.56µs ± 0%    -0.31%
RandomEncodeAllDefault-16               2.58ms ± 0%     2.52ms ± 0%    -2.20%
Random10MBEncoderFastest-16             3.61ms ± 0%     3.61ms ± 0%    -0.04%
RandomEncoderDefault-16                 3.44ms ± 0%     3.44ms ± 0%    +0.03%

name                                  old speed      new speed       delta
Encoder_EncodeAllXML-16                402MB/s ± 0%    410MB/s ± 0%    +2.16%
Encoder_EncodeAllSimple/fastest-16     173MB/s ± 0%    175MB/s ± 0%    +1.10%
Encoder_EncodeAllSimple/default-16     116MB/s ± 0%    107MB/s ± 0%    -7.52%
Encoder_EncodeAllSimple/better-16     99.0MB/s ± 0%  101.4MB/s ± 0%    +2.38%
Encoder_EncodeAllSimple/best-16       6.21MB/s ± 0%  14.61MB/s ± 0%  +135.27%  <====
Encoder_EncodeAllSimple4K/fastest-16  1.52GB/s ± 0%   1.60GB/s ± 0%    +5.56%
Encoder_EncodeAllSimple4K/default-16   124MB/s ± 0%    122MB/s ± 0%    -1.29%
Encoder_EncodeAllSimple4K/better-16    104MB/s ± 0%    106MB/s ± 0%    +1.13%
Encoder_EncodeAllSimple4K/best-16     5.59MB/s ± 0%  11.39MB/s ± 0%  +103.76%  <====
Encoder_EncodeAllHTML-16               208MB/s ± 0%    213MB/s ± 0%    +2.11%
Encoder_EncodeAllTwain-16              120MB/s ± 0%    120MB/s ± 0%    +0.04%
Encoder_EncodeAllPi-16                89.0MB/s ± 0%   89.9MB/s ± 0%    +1.02%
Random4KEncodeAllFastest-16           4.14GB/s ± 0%   4.20GB/s ± 0%    +1.32%
Random10MBEncodeAllFastest-16         4.19GB/s ± 0%   4.22GB/s ± 0%    +0.71%
Random4KEncodeAllDefault-16            895MB/s ± 0%    897MB/s ± 0%    +0.31%
RandomEncodeAllDefault-16             4.06GB/s ± 0%   4.15GB/s ± 0%    +2.25%
Random10MBEncoderFastest-16           2.90GB/s ± 0%   2.90GB/s ± 0%    +0.04%
RandomEncoderDefault-16               3.05GB/s ± 0%   3.05GB/s ± 0%    -0.03%
```

* Add comment

zstd/enc_best.go

@@ -32,6 +32,7 @@ type match struct {
 	length int32
 	rep    int32
 	est    int32
+	_      [12]byte // Aligned size to cache line: 4+4+4+4+4 bytes + 12 bytes padding = 32 bytes
 }
 
 const highScore = 25000