Mix-side exported interfaces are unsound

[Drup]

Let's consider a .cmi exporting this api:

val f : unit -> Dom.t Fragment.t

If this is loaded by vanilla ocaml Dom.t will not be considered client, and the symbol might not even exist. Fragment.t should still be a black box, though.

@vouillon proposed to emit a different magic for such cmi.

[Drup]

This is now done (by, indeed, emitting cmi files with different magic).