External objects constructor/deconstructor should use external functions

[Martin Sjölund]

The following should work (and does if called as an external function: MyExtObj.constructor({1,2,3,4,5})... which is disallowed...).

function constructor
  input Real r[:];
  output MyExtObj obj;
external "C" obj = myConstructor(r,size(r,1),{1,2,3,4},"abc");
end constructor;

Also, we should probably push/pop external objects to a stack. Right now, we only support construction of external objects during initialization :)