rc_buffer lint is dangerous as it changes runtime behaviour

clippy warns if one tries to use a type like Rc<Vec<T>>, e.g.

Arc<Vec<u8>>
^^^^^^^^^^^^ help: try: `Arc<[u8]>`

This is dangerous as creating a Rc<[T]> causes a memcpy of the whole memory area with all the elements, while creating an Rc<Vec<T>> simply copies over the 3 pointers inside the Vec without copying all the elements.

A better suggestion would probably be Rc<Box<[T]>> as that has the same runtime behaviour, but even independent of that it can be useful to have a Vec inside an Rc / Arc in combination with the make_mut() / get_mut() API.

The lint should probably be at least disabled by default.

The same also applies to Rc<String> vs Rc<str>.

Start by reading the rc_buffer lint implementation and its existing tests, then reproduce the Arc<Vec> and Rc cases described in the issue. Compare the suggested replacements with Vec/String mutation behavior and make the lint's behavior match the intended runtime semantics; update coverage so the relevant cases are verified.