[META] Opaque items and behaviour of interface-only mode

[maximebuyse]

While trying to allow opaque impls and functions in #1119, it became clear that this is very linked to what happens in interface-only mode.
We wrote a detailed design for both to unify them and make sure they are coherent and have a useful and understandable behavior. In particular this document describes, for each kind of item, what it means to be erased. All items marked opaque by the user are erased and for interface-only modules, certain kinds of items are also automatically erased.