Lint against Rc<String> (and Rc<Vec<T>>)

Now that it's easy to construct `Rc<str>` and `Rc<[T]>` using `into()`, we should advise people to use them.

Especially `Rc<String>` is often advised to avoid cloning strings, but has no advantages (except if you want to use `get_mut`) and is a) larger, b) has one more pointer to chase, and c) doesn't implement `Borrow<str>`.