[Design] HonkPerf.NET.GenericSIMD

# GenericSIMD

We're probably going to use Silk.NET.Maths for `Simd128` and `Simd256` sets. The API itself though would rely on `Span<T>`.

Current thought:
```cs
static void Add<T>(Span<T> a, Span<T> b, Span<T> c)
```