Change the representation of export lists in .hi files

Currently export list in .hi files are partitioned by module
  export M T(C1,C2)
         N f,g
In each list we only have OccNames, all assumed to come from
the parent module M or N resp.

This patch changes the representatation so that export lists
have full Names:
  export M.T(M.C1,M.C2), N.f, N.g

Numerous advatages
  * AvailInfo no longer needs to be parameterised; it always
    contains Names

  * Fixes Trac #5306.  This was the main provocation

  * Less to-and-fro conversion when reading interface files

It's all generally simpler.  Interface files should not get bigger,
becuase they have a nice compact representation for Names.