Add new CRC-32C checksum functions to folly

Summary:
* Added a new crc32c() function, with a portable implementation
and an optimized version for x86 with SSE4.2

Test Plan: New unit test included

Reviewed By: tudorb@fb.com

FB internal diff: D1111515
Remove FOLLY_NORETURN from definition.

Summary:
- Remove FOLLY_NORETURN from function definitions, which was causing
compilation errors on some platforms.

Test Plan: - Compile on Mac OS X with Clang

Reviewed By: tudorb@fb.com

FB internal diff: D1111416