Function parallelhash256_xof 

pub fn parallelhash256_xof(
    data: &[u8],
    block_size: usize,
    custom: &[u8],
    out: &mut [u8],
)

ParallelHashXOF256 (NIST SP 800-185 §6).

XOF variant: right_encode(0) for the final length field. NIST SP 800-185 §6.3.1.