Added ocaml-vg-0.8.1 and deps ocaml-otfm-0.2.0 and ocaml-gg-0.9.0

NB: Loading the HTML canvas backend of Vg crashes with `Unimplemented Javascript primitive caml_js_var!`. This seems to be a bug with the Nixpkgs packaging of js_of_ocaml, since it can be reproduced by just trying to load js_of_ocaml into the Ocaml toplevel.