Make it enable to take &PyClass as arguments as pyfunctions/methods

[kngwyu]

Resolves #786.
For C: PyClass, this PR enables the use of &C, &mut C, Option<&C> and Option<&mut C> as function/method arguments.
The hack I used is

1. Set ExtractExt::Target for &C, like

impl<'a> pyo3::derive_utils::ExtractExt<'a> for &'a C
{
    type Target = pyo3::PyRef<'a, C>;
}

2. If the argument is one of &C/&mut C/Option<&C>/Option<&mut C>, extract the argument to <type as ExtractExt::Target> and insert deref.

But, for object protocols, still we need to use PyRef<C> or PyRefMut<C> due to its trait requirements.

[kngwyu]

This is just a refactoring.
Not related to the logic so much.

[kngwyu]

Somehow tests/test_dunder.rs was mode 100755 so I executed chmod -x for that file.

[davidhewitt]

LGTM. One small nitpick on the test.

[kngwyu]

Thank you for the review 😉

[1tgr]

FYI, I think the syntax x: &Self no longer works for method parameters as a result of this change. A method taking a parameter of type &Self did work on pyo3 v0.8.5.

It's because of this usage of the #tref token outside of the class's impl block:
https://github.com/kngwyu/pyo3/blob/96115eaaaad6df8039fb0dc07521137a89ce15b4/pyo3-derive-backend/src/pymethod.rs#L567

[davidhewitt]

Thanks, have opened #1070 to remember this.