Function assemble_statement 

pub fn assemble_statement(
    signing_payload: &[u8],
    num_fields: u32,
    sr25519_pubkey: &[u8; 32],
    signature: &[u8; 64],
) -> Vec<u8> 

Assemble a complete encoded statement from a signing payload and sr25519 signature.

signing_payload and num_fields must come from build_signing_payload.