Function avx2_decode 

unsafe fn avx2_decode(
    kind: SimdKind,
    input: &[u8],
    quads_end: usize,
    output: &mut [u8],
) -> (usize, usize)

Dispatch the AVX2 decode kernel to the right alphabet monomorphization.

Safety

The running CPU must support AVX2 (the requirement is forwarded to avx2::decode_bulk).