Handle instantiation of builtin functions better in new instantiation

[Per Östlund]

In the new instantiation we currently handle builtin function calls by checking if the function we're looking for is one of the known builtin functions. This is not correct, since it makes it impossible to redefine the builtin functions. We should instead look them up in the environment, and ideally instantiate them as normal functions. Some of the builtin functions might need special handling, but I think it should be possible to define all builtin functions with overload and __OpenModelica_varArgs, at least for the purpose of instantiation (typing might be another matter).