function_traits for std::bind

[robiwano]

Good stuff! I plan on using function_traits for a OSC server. It seems it cannot (as of yet) resolve for std::bind:

struct Foo {
    void test_fn(int a1, double a2) {}
};

Foo foo;
auto fn = std::bind(&Foo::test_fn,&foo,std::placeholders::_1, std::placeholders::_2);
std::cout << utils::function_traits<decltype(fn)>::arity << std::endl; // << doesn't compile

[robiwano]

I see now that the test for std::bind is commented out. https://github.com/kennytm/utils/blob/master/tests/traits.cpp#L157