Dynlink with Findlib support for long-running plugins

In the config file

```toml
[plugins]
   ocamlfind = ["some-package", "some-other-package"]
   compiled = "some_dyn_compiled_module.cmxs"
```

Ketrew will load at runtime the findlib packages and the compiled module.

There is one example of plugin: `src/test/dummy_plugin.ml`  and a workflow that uses it `dummy_plugin_user.ml`.

Documentation will come in a later PR (other things to document).

Issue #1 is known already; saved for later.





The Error monad with polymorphic variants is complex enough to scare contributors; so I removed the phantom-powered paths (the error monad is useful, the `Path` module was … a toy :) ).