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.
That would work but it won't necessary be optimized out. This will be forwards compatible because the basic properties of strings / vectors are going to end up as guarantees for `unsafe` code.