Import OCaml library Biniou into wip/ocaml-biniou.

Biniou (pronounced "be new") is a binary data format designed for
speed, safety, ease of use and backward compatibility as protocols
evolve. Biniou is vastly equivalent to JSON in terms of functionality
but allows implementations several times faster (4 times faster than
yojson), with 25-35% space savings.
Add ocaml-biniou.