Allow first argument in #[cgp_getter] methods to be non-self

[soareschen]

Summary

This PR improves the #[cgp_getter] macro to allow values other than self to be the receiver of the first argument in the getter method.

For example, the following is now allowed:

#[cgp_getter]
pub trait HasFooBar: HasFooType + HasBarType {
    fn foo_bar(foo: &Self::Foo) -> &Self::Bar;
}