This used to conflict with the util crate from the standard library, which
has long since been removed.
The import in layout has not been changed because of a conflict with the
util mod there.
This gets rid of a dubious transmute:
let val = mem::transmute::<&RefCell<T>, &T>(&self.base);
The code duplication will be reduced once rust-lang/rust#18131 is fixed.
This type simply wraps `RefCell<T>` to add the special method,
and introduce the method to return the pointer of the value contained in
itself, for used in layout task.