Exporting wrapped functions doesn't work

I find myself in need of exporting a whole lot of functions to Python which require a wrapper (the API needs locking and whatnot).

The following code probably should work (pybind11 can handle it, for the most part), but doesn't.

```
#include
#include

template
struct wrap
{
using funct_type = R(*)(Args...);
funct_type func;
wrap(funct_type f): func(f) {};
R operator()(Args&&... args)
{
std::cout << "before calling\n";
R ret=func(std::forward(args)...);
std::cout << "After calling\n";
return ret;
}
};

class bar {};
class foo : bar {
public:
enum xfoo { FOO=1, BAR=2, BAZ=3 };
static int add(int a, int b, enum xfoo c) { return a+b; }
static int add2(int a, int b) { return a+b; }
};

BOOST_PYTHON_MODULE(example)
{
using namespace boost::python;

// this works
std::cout << "Result" << wrap{foo::add}(1,2, foo::xfoo::FOO) << std::endl;

// so does this
def("add", foo::add, "A function that adds two numbers and no enum");

// this works with pybind11 but not boost
def("add2", wrap{foo::add2}, "A function that adds two numbers");

// this works with neither
def("add", wrap{foo::add}, "A function that adds two numbers and no enum");
}
```

Start by reproducing the wrapped-function examples in the issue, then inspect the boost::python::def entry point and its handling of callable objects. Done means the wrapped add2 and add examples can be exported successfully, matching the cases that already work directly.

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