Add support for core.simd, intel-intrinsics, or inlined assembly

[dd86k]

Waiting on: #2

Options:

1. core.simd -- Supported everywhere, I think.
2. intel-intrinsics DUB package -- Somewhat supports all compilers.
3. Inlined assembly -- If all fails, at least x86 users would benefit. But limited to AVX/AV2 and not SSE* at best (because DMD).

Versions: Blake2dUseSIMD or Blake2dUseIntrinsics (user will have to manually assign version)