std/crypto: add a vectorized ChaCha20 implementation
std/crypto: add a vectorized ChaCha20 implementation

Brings a 30% speed boost on x86_64 even though we still process only
one block at a time for now.

Only enabled on x86_64 since the non-vectorized implementation seems
to currently perform better on some architectures (at least on aarch64).

But the non-vectorized implementation still gets a little speed boost
as well (~17%) with these changes.

andrewrk/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig