feature request, Ref::map, RefMut::map

same feature with std::cell::Ref::map, std::cell::RefMut::map

ex)
```

struct Foo {
bar:i32
}

let cell = RefCell::new(Foo { bar: 1 });

let cell_ref = cell.borrow();
let cell_ref_bar = Ref::map(cell_ref, |v| &v.bar);

```