XL/bitrot: Migrate to new blake2b-simd SIMD optimized implementation. (#2031)
Thanks for Frank Wessels <fwessels@xs4all.nl> for all the heavy lifting work.
Comparative benchmarks are as below.
```
benchmark old ns/op new ns/op delta
BenchmarkHash64-4 742 411 -44.61%
BenchmarkHash128-4 681 346 -49.19%
BenchmarkWrite1K-4 4239 1497 -64.69%
BenchmarkWrite8K-4 33633 11514 -65.77%
BenchmarkWrite32K-4 134091 45947 -65.73%
BenchmarkWrite128K-4 537976 183643 -65.86%
benchmark old MB/s new MB/s speedup
BenchmarkHash64-4 86.18 155.51 1.80x
BenchmarkHash128-4 187.96 369.10 1.96x
BenchmarkWrite1K-4 241.55 683.87 2.83x
BenchmarkWrite8K-4 3897.06 11383.41 2.92x
BenchmarkWrite32K-4 977.48 2852.63 2.92x
BenchmarkWrite128K-4 243.64 713.73 2.93x
```
Fixes #2030
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