Function btck_block_to_bytes 

pub unsafe extern "C" fn btck_block_to_bytes(
    block: *const btck_Block,
    writer: btck_WriteBytes,
    user_data: *mut c_void,
) -> c_int

@brief Serializes the block through the passed in callback to bytes. This is consensus serialization that is also used for the P2P network.

@param[in] block Non-null. @param[in] writer Non-null, callback to a write bytes function. @param[in] user_data Holds a user-defined opaque structure that will be passed back through the writer callback. @return 0 on success.