Implemented Crc32

Please check if the 32 bits path is being done properly. I only tested it with 64 bits.

Tests run: 
```bash
PYTHONPATH=~/test_hippy/pypy py.test -r s test_phpt/ext/standard/strings/crc32* test_phpt/ext/standard/strings/bug36306.phpt
```

Most of the tests are for 32 bits. However, `test_phpt/ext/standard/strings/crc32_basic.phpt` and  `test_phpt/ext/standard/strings/crc32_error.phpt` can be perfectly run in 64 bits (and they pass!).