I have run into issues with an opam switch (4.03.0) where ppx_deriving 4.1.5
was installed. I would get an error message from dune claiming that
visitors.ppx cannot be used ("Error: no ppx driver were found").
Upgrading to ppx_deriving 4.4 fixed the problem.

which were unused and are broken by OCaml 4.08.

This used to be provided by ppx_deriving as a transitive dependency,
but now that ppx_deriving switched to Dune the break of "visitors"
breaks in an empty environment.

Tweak the build to make everything work when using visitors with dune

See merge request !3

This seems to be useful so that dune/jbuilder automatically loads the runtime
library

This seems to be required for the ppx to work when using dune/jbuilder

Add negative test cases where a warning should be issued (and the generated code is rejected by OCaml).