Function build_resp03 

pub fn build_resp03<const MAX_QTY: usize, R: RegisterRead>(
    out: &mut [u8],
    unit_id: u8,
    start_addr: u16,
    quantity: u16,
    regs: &R,
) -> usize

组装 03 响应帧：Unit(1)+Func(1)+ByteCount(1)+Reg(2*quantity)+CRC(2)