add #[experimental] as_string/as_vec functions

This provides a way to pass `&[T]` to functions taking `&U` where `U` is
a `Vec<T>`. This is useful in many cases not covered by the Equiv trait
or methods like `find_with` on TreeMap.
A more forward-compatible way of doing this might be

    mem::forget(mem::replace(self, Vec::new)).