Support abstract functions with simple return types

[NTillmann]

I need this to work:

let f = __abstract(":object", "(__f)");
global.result = f().p;

But it currently fails because the access to .p is not known to be safe. I'd like to model that the result of the call to f is a simple object where all such accesses should simply be allowed, maybe via __abstract(":object(simple)", ...).