expose_functions: warning when function uses reserved word?

[WardBrian]

Because of the need to compile as valid C++, there is sometimes name mangling applied by the stanc compiler. This can affect both function names and function argument names, and I believe was the cause of epinowcast/primarycensored#258

For example:

functions {
  real min(real max, real class) {
    return max - class;
  }
}

Generates the following

// [[stan::function]]
double
_stan_min(const double& _stan_max, const double& _stan_class, std::ostream*
          pstream__ = nullptr) {
  return reserved_model_namespace::_stan_min(_stan_max, _stan_class,
           pstream__);
}

The function name could be fixed by adding the optional ("name") to the Rcpp:Export comment, but I don't think anything can be done about the argument names. Maybe cmdstanr should detect this _stan prefix and give a warning?