Impossible to provide custom pushforwards for functions that use functors

[gojakuch]

In this case, the compilation fails:

#include "clad/Differentiator/Differentiator.h"
#include <iostream>

namespace clad::custom_derivatives {
template <typename F>
void use_functor_pushforward(double x, F& f, double d_x, F& d_f) {
    f.operator_call_pushforward(x, &d_f, d_x);
}
}

template <typename F>
void use_functor(double x, F& f) {
    f(x);
}

struct Foo {
    double &y;
    double operator()(double x) {
        y = 2*x;

        return x;
    }
};

double fn(double x) {
    Foo func {x};

    use_functor(x, func);

    return x;
}

int main() {
  auto df = clad::differentiate(fn, "x");
  std::cout << "f(3)=" << fn(3.0);
  std::cout << "; df(3)=" << df.execute(3.0) << '\n';
  return 0;
}

because at the time of parsing the custom derivative, the compiler doesn't know we're going to insert the operator_call_pushforward. we need to somehow bypass this in a user-friendly way, since this is a common pattern in modern frameworks like Kokkos.