Port ppx from omp to ppxlib

[NathanReb]

This PR ports obus.ppx from OMP to ppxlib. As discussed here, the next major release of OMP will drop significant parts of its API including the driver and Ast_mapper.

I did the naive port using Ast_traverse to keep this simple but I have the feeling this could be turned into a deriving ppx.
Maybe with a few changes to ppxlib we wouldn't even have to break the ppx's API and could keep the [@@obus ...] syntax. Rewriting as such would improve performances when obus.ppx is combined with other PPXes.

You'll note that I bumped the minimum requirement for dune. That is due to the fact the 2.0 menhir extension is only available from dune 1.4 forward.

[meadofpoetry]

Thanks.

I did the naive port using Ast_traverse to keep this simple but I have the feeling this could be turned into a deriving ppx.

Well, I'm planning to remove ppx in 2.0 (it seems like nobody uses it, at least no one has said they do), so one could write a totally new ppx for obus as a standalone library, e.g. as a ppx_deriving plugin.