David added the JamCRC implementation in r246590. More recently, Eugene added a CRC-32 implementation in r357901, which falls back to zlib's crc32 function if present.
These checksums are essentially the same, so having multiple implementations seems unnecessary. This replaces the CRC-32 implementation with the simpler one from JamCRC, and implements the JamCRC interface in terms of CRC-32 since this means it can use zlib's implementation when available, saving a few bytes and potentially making it faster.
JamCRC took an ArrayRef<char> argument, and CRC-32 took a StringRef. This patch changes it to ArrayRef<uint8_t> which I think is the best choice, and simplifies a few of the callers nicely.
Please take a look!