CHANGES:
- Migrate scrypt from Cstruct.t to string
- Merge ocaml-pbkdf (from https://github.com/abeaumont/ocaml-pbkdf),
hkdf (from https://github.com/hannesm/ocaml-hkdf), and scrypt (from
https://github.com/abeaumont/ocaml-scrypt-kdf) into a single repository
and opam package (with three subpackages, kdf.pbkdf. kdf.hkdf, and
kdf.scrypt). - Disable a failing testcase for architectures with integers no longer than 31
bits (thanks to @kit-ty-kate)