Function parallelhash256 

pub fn parallelhash256(
    data: &[u8],
    block_size: usize,
    custom: &[u8],
    out: &mut [u8],
)

ParallelHash256 (NIST SP 800-185 §6).

block_size is B in octets and must be non-zero. Each B-octet block is compressed to a PARALLELHASH_CV_256-octet chaining value. NIST SP 800-185 §6.3.