computes sha512 hash of a string
string sha512(string source)
sha512("a") /* 1F40FC92DA241694750979EE6CF582F2D5D7D28E18335DE05ABC54D0560E0F5302860C652BF08D56 0252AA5E74210546F369FBBBCE8C12CFC7957B2652FE9A75 */