Some performance comparisons to C.
We take the fastest time measurement taken across multiple runs.
The block hashing functions use the same md5/sha1 methods.
```
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-md5-hash-implementation-in-x86-assembly:
```
gcc -O2
661 Mb/s
clang -O2
490 Mb/s
zig --release-fast and zig --release-safe
570 Mb/s
zig
50 Mb/s
```
See https://www.nayuki.io/page/fast-sha1-hash-implementation-in-x86-assembly:
```
gcc -O2
588 Mb/s
clang -O2
563 Mb/s
zig --release-fast and zig --release-safe
610 Mb/s
zig
21 Mb/s
```
In short, zig provides pretty useful tools for writing this sort of
code. We are in the lead against clang (which uses the same LLVM
backend) with us being slower only against md5 with GCC.
