Add CRC32C variant and file processing API

[code-inflation]

Summary

Add new CRC32C variant and file processing capabilities with enhanced Hasher features.

New Features

• CRC32C support: Add crc32c() function using Castagnoli polynomial (common in iSCSI, Ethernet)
• File processing: Add crc32_file() and crc32_fileobj() functions for convenient file checksumming
• Enhanced Hasher:
  • copy() method for creating independent hasher copies
  • Context manager support (with Hasher() as h:)
  • __len__() method to track bytes processed
  • __repr__() method for debugging
• Improved error handling: Clear, helpful error messages for common usage mistakes
• Type safety: Full type hints with .pyi stub file for IDE support

API Changes

# New CRC32C variant
from pycrc32 import crc32c
result = crc32c(b"data")

# File processing
from pycrc32 import crc32_file, crc32_fileobj
file_crc = crc32_file("/path/to/file")
obj_crc = crc32_fileobj(file_object)

# Enhanced Hasher features
hasher = Hasher()
hasher.update(b"data")
print(len(hasher))  # Bytes processed
copy_hasher = hasher.copy()

with Hasher() as h:
    h.update(b"context")