functional: support bound methods

If a bound std::function is invoked with a bound method, the implicit
bound self is lost because we use `detail::get_function` to unbox the
function.  This commit amends the code to use py::function and only
unboxes in the special is-really-a-c-function case.  This makes bound
methods stay bound rather than unbinding them by forcing extraction of
the c function.

tests/test_callbacks.cpp

@@ -179,4 +179,9 @@ test_initializer callbacks([](py::module &m) {
         f(x); // lvalue reference shouldn't move out object
         return x.valid; // must still return `true`
       });
+
+    struct CppBoundMethodTest {};
+    py::class_<CppBoundMethodTest>(m, "CppBoundMethodTest")
+        .def(py::init<>())
+        .def("triple", [](CppBoundMethodTest &, int val) { return 3 * val; });
 });