Unique owner `Rc`, like `Box`, can `Send` and `DerefMut`
```rust
use std::{rc::Rc, thread::spawn};
use unique_rc::UniqRc;
let rc = Rc::new("foo".to_owned());
let weak = Rc::downgrade(&rc);
assert_eq!(weak.upgrade(), Some(Rc::new("foo".to_owned())));
let urc = UniqRc::new(rc);
assert_eq!(weak.upgrade(), None);
```
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