dfmc-modeling: Export more compile-time raw types

* sources/dfmc/modeling/modeling-library.dylan
  (module dfmc-modeling): Export <&raw-c-signed-int> and <&raw-c-pointer>.
Bug 7458: Properly emit null pointer literals

* sources/dfmc/llvm-back-end/llvm-emit-object.dylan
  (emit-object): Add a new method so that literals of type
   <&raw-pointer>, <&raw-c-pointer>, or <&raw-byte-string>
   can be output as LLVM null constants.
Snapshot