Unique owner Rc, like Box, can Send and DerefMut

Example

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);

spawn(move || {
    assert_eq!(*urc, "foo");
}).join().unwrap();

assert_eq!(weak.upgrade(), None);

Safety

• miri test passed