Function encode_rlp_list_header 

pub fn encode_rlp_list_header(
    payload_len: usize,
    output: &mut [u8],
) -> Result<usize, DecodeError>

Encodes only the RLP list header for a known payload length.

The payload must later be filled with canonical encoded child items. This helper is intended for no-allocation encoders that cannot first materialize the whole list payload in a separate buffer.