Gc<RefCell<T>> can be more ergonomic

[Others]

If you have a Gc<RefCell<T>> you need to have two guards:

let gc_guard = gc.get();
let ref_guard = gc_guard.borrow_mut();

Instead we should have some sort of impl like:

impl<T: Scan> Gc<RefCell<T>> {
    fn borrow() -> GcRef {
        ...
    }

    fn borrow_mut() -> GcRefMut {
        ...
    }
}

Which would increase ergonomics! Unfortunately GcRef and GcRefMut would be self referential structs, so we'd probably need rental, and that's why I haven't done this yet.