VHDL语言实现的CRC校验，函数形式，包括CRC4,CRC8,CRC16和CRC32

-- crc.vhd -- Used for calculation of CRC16-CCITT -- Intended use is as custom peripheral for Nios processor -- When address is logic 0 => -- Internal CRC register is initialised with write_data value -- When address is logic 1 =>