No shared conversions for other string-like types (Path, OsStr, CStr)

Conversions for `Rc<str>`, `Arc<str>`, `Rc<[T]>`, and `Arc<[T]>` were added in #42565, but it is still not possible to make a shared container for other string-like types - you still need to use double allocated containers like `Rc<PathBuf>`. It looks like the only types this is missing for are `Path`, `OsStr`, and `CStr`.