We take the fastest time measurement taken across multiple runs. Tested
across multiple compiler flags and the best chosen.

```
Cpu: Intel(R) Core(TM) i5-6500 CPU @ 3.20GHz
Gcc: 7.2.1 20171224
Clang: 5.0.1
Zig: 0.1.1.304f6f1d
```

See https://www.nayuki.io/page/fast-sha2-hashes-in-x86-assembly.

```
Gcc -O2
    219 Mb/s
Clang -O2
    213 Mb/s
Zig --release-fast
    284 Mb/s
Zig --release-safe
    211 Mb/s
Zig
    6 Mb/s
```

```
Gcc -O2
    350 Mb/s
Clang -O2
    354 Mb/s
Zig --release-fast
    426 Mb/s
Zig --release-safe
    300 Mb/s
Zig
    11 Mb/s
```