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.
This `transmute` isn't safe right now (I can't imagine much that could go wrong though).

I think to fix that one would have to reimplement `AsVec` here.