Add support for `std::vector::size()`

[gojakuch]

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

double fn(double x, double y) {
    std::vector<double> v;
    for (size_t i = 0; i < v.size(); ++i) {
        v.push_back(x);
    }
    return v[0];
}

int main(int argc, char* argv[]) {
    double dx, dy;
    auto df = clad::gradient(fn, "x, y");
    std::cout << fn(3, 4) << '\n';
    dx = 0; dy = 0;
    df.execute(3, 4, &dx, &dy);
    std::cout << dx << ' ' << dy << '\n';
}

this code currently crashes because of the v.size() usage.