Change Ojs_exn to Ojs.Exn

[rgrinberg]

It's recommended that every library has a single toplevel module, and (wrapped false) is only there to help people transition to dune. Would it be ok to break compatibility and to switch to Ojs.Exn? If not, then perhaps we could add something to dune to help the transition from (wrapped false) to (wrapped true). We already have one such mechanism (wrapped (transition "message")) but it doesn't quite work in some cases (like this one)

[hhugo]

Why doesn't the "transition" mechanism work in that case ?

[rgrinberg]

In this case, Ojs_exn depends on Ojs. So we'd need to extract Ojs.t to some other module core, and then write ojs.ml as:

include Core
module Exn = Exn

But that would mean that (wrapped (transition ..)) would create a deprecated alias of the form ojs_core. This seems unnecessary.

I suppose that in this case we can just inline ojs_exn into ojs and then write ojs_exn as:

include Ojs.Exn

With a deprecation message. That seems alright to me.