Summary:
Added LtHash, a cryptographic homomorphic hash, to folly/experimental/crypto.
This has a soft dependency on libsodium and the code will not be compiled if libsodium is not detected by cmake.

Reviewed By: djwatson

Differential Revision: D13390825

fbshipit-source-id: f7597ced7bcc7b403e8bbaa733837b795128f1b3