[FR] UDFs taking another UDF as argument

A bit of a nice-to-have for simplifying more complex models would be the ability to pass a functions as an argument to another function.

With suggested signatures like:

functions {
  real mult(int x, real y) { return x * y; }
  real div(int x, real y) { return x / y; }
 
  real calc((int, real): real functor, int x, real y) {
    return functor(x, y);
  }
}

And usage:

int a = 1;
real b = 2;

real calc_mult = calc(mult, a, b);
real calc_div = calc(div, a, b);

And if it's not too tricky an additional request, being able to declare/assign functions would be great as well:

(int, real): real functor;

if (operation == 1) {
  functor = mult;
} else {
  functor = div;
}

real result = calc(functor, a, b);

Note that the above is a highly contrived and simplified example

The issue names no files, tests, or entry points. Start by locating how stanc3 parses and represents user-defined functions and function signatures, then determine the scope needed for passing and assigning functions. Done means the proposed examples are accepted and compile with the intended behavior, with tests covering both forms.