Pure Python CRC library.

Calculate CRC checksums, verify CRC checksum, predefined CRC configurations,
and custom CRC configurations.

Features:
* Calculate CRC8, CRC16, CRC32, and CRC64 checksums
* Verify CRC checksums
* Pre-defined configurations for common CRC algorithms (CCITT, XMODEM, etc.)
* Custom configurations support
* Optimized table-based calculations for speed
* Command-line interface for CRC calculations