generated code should use full path to PtrEq

[comex]

Currently, you cannot use #[derive(ptr_eq::PtrEq)] without also useing the trait. This is because when the proc macro generates unsafe impl #impl_gn PtrEq, PtrEq is not qualified. It should use a full module path.

Also, thanks for the crate!

[wx-csy]

Thank you for watching this crate!

I'm going to submit an RFC/PR for changing the related trait implementations of Rc, Arc, etc., in the standard library. The std lib Rc<T> should be changed to implement Hash where &T: Hash (instead of T: Hash). Then all we need is to write a derive macro that implements Hash<&T> for T.

The related discussions can be seen at https://internals.rust-lang.org/t/proposal-compare-rc-t-and-arc-t-by-t/14437.