added deps in dune-project and opam file under version control
This makes it easy to use local switches to compile/install plantinator:
opam switch create .
One the one hand it's not so great to have the opam file under version control (since it's generated, and might then be out-of-sync), but it's standard practice AFAIK and helps (a bit) with installability. If it's too undesirable, I can commit the deps only.