Add a scope guard call policy

```c++
m.def("foo", foo, py::call_guard<T>());
```

is equivalent to:

```c++
m.def("foo", [](args...) {
    T scope_guard;
    return foo(args...); // forwarded arguments
});
```

tests/CMakeLists.txt

@@ -28,6 +28,7 @@ endif()
 set(PYBIND11_TEST_FILES
   test_alias_initialization.cpp
   test_buffers.cpp
+  test_call_policies.cpp
   test_callbacks.cpp
   test_chrono.cpp
   test_class_args.cpp
@@ -40,7 +41,6 @@ set(PYBIND11_TEST_FILES
   test_exceptions.cpp
   test_inheritance.cpp
   test_issues.cpp
-  test_keep_alive.cpp
   test_kwargs_and_defaults.cpp
   test_methods_and_attributes.cpp
   test_modules.cpp