larger values of crc_len can cause a MemoryError

[robberwick]

In CRC.__init__(), the crc_len parameter is used to determine the length of the cs_table property, which is set as 2 ** crc_len. The entire list is then allocated in memory with the line

self.cs_table = [' '] * self.table_len

As the whole list needs to be in memory, this causes issue for even moderately large values of crc_len. On my machine (32GB ram), attempting to instantiate a crc instance with a crc_len of 32 hangs for a number of seconds before raising a MemoryError.

To resolve this, the pre-calculation of that cs_table could be deferred and performed lazily for each desired value, with that value being cached to prevent re-calculation. I would suggest not undertaking this work until #89 has been addressed, so that the potential for regressions can be reduced.