Some changes to the way FFI decls are handled:

  - a foreign export dynamic which returns a newtype of
    an Addr now works correctly.  Similarly for foreign label.

  - unlifted types are not allowed in the arguments of a foreign
    export.  Previously we generated incorrect code for these cases.

Newtypes in FFI declarations now work everywhere they should, as far
as I can see.

These changes will be backported into 4.08.1.