Summary:
This diff imports farmhash.h and farmhash.cc from
https://github.com/google/farmhash and updates the public_tld license
file accordingly. Build integration and namespace changes will occur in
later diffs.

Reviewed By: yfeldblum

Differential Revision: D13436553

fbshipit-source-id: 7a081032cb35a1a3e1cd14e2edf2685906956396