Import OCaml library comparelib into wip/ocaml-comparelib.

Pa_compare is a camlp4 syntax extension that derives comparison
functions from type representations. The scaffolded functions are
usually much faster than ocaml's Pervasives.compare. Scaffolding
functions also gives you more flexibilty by allowing you to override
them for a specific type and more safety by making sure that you only
compare comparable values.
Add ocaml-comparelib.